fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh()

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "fs/fhandle.c",
                        "fs/mount.h",
                        "fs/namespace.c"
                    ],
                    "versions": [
                        {
                            "version": "620c266f394932e5decc4b34683a75dfc59dc2f4",
                            "lessThan": "15ea8dc42a02259d49dee38a658d40f60fcd75ed",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "620c266f394932e5decc4b34683a75dfc59dc2f4",
                            "lessThan": "32138633e51e6db59e474765cf93268c92b42888",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "620c266f394932e5decc4b34683a75dfc59dc2f4",
                            "lessThan": "a8ed2c29fcfdac78db96c9da4e659c8a513f2a94",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "620c266f394932e5decc4b34683a75dfc59dc2f4",
                            "lessThan": "40ab6644b99685755f740b872c00ef40d9aa870e",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "fs/fhandle.c",
                        "fs/mount.h",
                        "fs/namespace.c"
                    ],
                    "versions": [
                        {
                            "version": "6.11",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "6.11",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12.95",
                            "lessThanOrEqual": "6.12.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.18.36",
                            "lessThanOrEqual": "6.18.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.0.13",
                            "lessThanOrEqual": "7.0.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.1",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }