NFSD: Fix potential use-after-free in nfsd_file_put()

nfsd_file_put_noref() can free @nf, so don't dereference @nf
immediately upon return from nfsd_file_put_noref().

Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Fixes: 99939792

 ("nfsd: Clean up nfsd_file_put()")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>