net: cap advertised IP tunnel headroom

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "include/net/ip_tunnels.h",
                        "net/ipv4/ip_tunnel.c",
                        "net/ipv6/ip6_gre.c",
                        "net/ipv6/ip6_tunnel.c",
                        "net/ipv6/sit.c"
                    ],
                    "versions": [
                        {
                            "version": "1a37e412a0225fcba5587f24c0dfc7636efc8b69",
                            "lessThan": "fb889a619723032140f5d983a3a34d25a5a2bed4",
                            "status": "affected",
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                        },
                        {
                            "version": "1a37e412a0225fcba5587f24c0dfc7636efc8b69",
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                        },
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                            "status": "affected",
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                        },
                        {
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                        },
                        {
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                            "status": "affected",
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                        },
                        {
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                        },
                        {
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                        {
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                            "status": "affected",
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                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "include/net/ip_tunnels.h",
                        "net/ipv4/ip_tunnel.c",
                        "net/ipv6/ip6_gre.c",
                        "net/ipv6/ip6_tunnel.c",
                        "net/ipv6/sit.c"
                    ],
                    "versions": [
                        {
                            "version": "3.11",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "3.11",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.10.270",
                            "lessThanOrEqual": "5.10.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.15.221",
                            "lessThanOrEqual": "5.15.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.188",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.157",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12.109",
                            "lessThanOrEqual": "6.12.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.18.50",
                            "lessThanOrEqual": "6.18.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.2.4",
                            "lessThanOrEqual": "7.2.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.3",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }