lockd: Use xdrgen XDR functions for the NLMv4 LOCK_MSG procedure

lockd: Use xdrgen XDR functions for the NLMv4 LOCK_MSG procedure

The LOCK_MSG procedure is part of NLM's asynchronous lock
request flow, where clients send LOCK_MSG to request locks
that may block. This patch continues the xdrgen migration by
converting LOCK_MSG to use generated XDR functions.

This patch converts the LOCK_MSG procedure to use xdrgen
functions nlm4_svc_decode_nlm4_lockargs and
nlm4_svc_encode_void generated from the NLM version 4
protocol specification. The procedure handler uses xdrgen
types through the nlm4_lockargs_wrapper structure that
bridges between generated code and the legacy nlm_lock
representation.

The pc_argzero field is set to zero because xdrgen decoders
reliably initialize all arguments in the argp->xdrgen field,
making the early defensive memset unnecessary. Remaining
argp fields are cleared as needed.

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

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