nfsd: clean up potential nfsd_file refcount leaks in COPY codepath

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                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "fs/nfsd/nfs4proc.c"
                    ],
                    "versions": [
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                        },
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                            "version": "6.3",
                            "lessThanOrEqual": "*",
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