nfsd: Clean up nfsd_file_put()

Make it a little less racy, by removing the refcount_read() test. Then
remove the redundant 'is_hashed' variable.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>