NFSD: Refactor nfsd4_encode_lock_denied()

Use the modern XDR utility functions.

The LOCK and LOCKT encoder functions need to return nfserr_denied
when a lock is denied, but nfsd4_encode_lock4denied() should return
a status code that is consistent with other XDR encoders.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>