nfsd_file_put_noref() can free @nf, so don't dereference @nf
immediately upon return from nfsd_file_put_noref().
Suggested-by: NTrond Myklebust <trondmy@hammerspace.com>
Fixes: 99939792 ("nfsd: Clean up nfsd_file_put()")
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>