lockd: Use xdrgen XDR functions for the NLMv4 UNLOCK procedure

lockd: Use xdrgen XDR functions for the NLMv4 UNLOCK procedure

UNLOCK releases locks acquired via the LOCK procedure. Conversion
of TEST, LOCK, CANCEL, and UNLOCK provides the complete set of
lock lifecycle operations required by the NLM protocol, enabling
clients to test for conflicts, acquire locks, abort pending lock
requests, and release held locks.

The procedure handler converts arguments from the xdrgen-generated
nlm4_unlockargs structure to the legacy nlm_lock representation
through nlm4_unlockargs_wrapper. This maintains compatibility with
core lockd logic while using XDR decoders and encoders generated
from the NLMv4 protocol specification.

The original __nlm4svc_proc_unlock function is retained because
the asynchronous callback path invokes it directly, bypassing
the RPC dispatch mechanism.

The pc_argzero field is zero because nlm4_svc_decode_nlm4_unlockargs
initializes all fields in argp->xdrgen, eliminating the need for
early memset of the argument buffer. Remaining argp fields outside
the xdrgen structure are cleared explicitly where needed.

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