nfsd: guard nfsd_serv deref in nfsd_file_net_dispose

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "fs/nfsd/filecache.c"
                    ],
                    "versions": [
                        {
                            "version": "ffb402596147ac583f3464ff5c48feb9423e3838",
                            "lessThan": "f4776c1c4b38fbc459420321c8ece87d0f7f95fb",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "ffb402596147ac583f3464ff5c48feb9423e3838",
                            "lessThan": "59baf45a06435194005fc5fa9a42d89f77a30432",
                            "status": "affected",
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                        },
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                        },
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                            "version": "ffb402596147ac583f3464ff5c48feb9423e3838",
                            "lessThan": "9f1ddfc8cb9076592401a611eb3a44d36186d014",
                            "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/nfsd/filecache.c"
                    ],
                    "versions": [
                        {
                            "version": "6.9",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "6.9",
                            "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"
                        }
                    ]
                }