lockd: Use xdrgen XDR functions for the NLMv4 UNLOCK_MSG procedure

lockd: Use xdrgen XDR functions for the NLMv4 UNLOCK_MSG procedure

Convert the UNLOCK_MSG procedure to use xdrgen functions
nlm4_svc_decode_nlm4_unlockargs and nlm4_svc_encode_void.
The procedure handler uses the nlm4_unlockargs_wrapper
structure that bridges between xdrgen types and the legacy
nlm_lock representation.

The pc_argzero field is set to zero because xdrgen decoders
reliably initialize all arguments, making the early
defensive memset unnecessary.

The NLM async callback mechanism uses client-side functions
which continue to take legacy struct nlm_res, preventing
UNLOCK and UNLOCK_MSG from sharing code for now.

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