l2tp: prevent possible tunnel refcount underflow

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/l2tp/l2tp_core.c",
                        "net/l2tp/l2tp_core.h",
                        "net/l2tp/l2tp_netlink.c",
                        "net/l2tp/l2tp_ppp.c"
                    ],
                    "versions": [
                        {
                            "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c",
                            "lessThan": "aa7132362e79abc4f2add4d8657dd1679341edc5",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c",
                            "lessThan": "98691b2e28a6c3dac0680914493d54c77f215755",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c",
                            "lessThan": "f7415e60c25a6108cd7955a20b2e66b6251ffe02",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c",
                            "lessThan": "24256415d18695b46da06c93135f5b51c548b950",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "b102bfc2a90d14f342580285782a9a51c74f7369",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "10c15ddabbcf888922adbdd44ca3fecf6eab19d9",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "8d1c650d452c53fcb3f02a7b1d772741639f89a4",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "12b5fb58ac993c24210cf8cbc72d407d3a4e6490",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "aef37401b467a0b1a9517c69924a1d66937e0789",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "3.2.99",
                            "lessThan": "3.3",
                            "status": "affected",
                            "versionType": "semver"
                        },
                        {
                            "version": "3.16.54",
                            "lessThan": "3.17",
                            "status": "affected",
                            "versionType": "semver"
                        },
                        {
                            "version": "4.4.225",
                            "lessThan": "4.5",
                            "status": "affected",
                            "versionType": "semver"
                        },
                        {
                            "version": "4.9.225",
                            "lessThan": "4.10",
                            "status": "affected",
                            "versionType": "semver"
                        },
                        {
                            "version": "4.14.182",
                            "lessThan": "4.15",
                            "status": "affected",
                            "versionType": "semver"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/l2tp/l2tp_core.c",
                        "net/l2tp/l2tp_core.h",
                        "net/l2tp/l2tp_netlink.c",
                        "net/l2tp/l2tp_ppp.c"
                    ],
                    "versions": [
                        {
                            "version": "4.15",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.15",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.191",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.160",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.11.3",
                            "lessThanOrEqual": "6.11.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }