lockd: Use xdrgen XDR functions for the NLMv4 CANCEL procedure

lockd: Use xdrgen XDR functions for the NLMv4 CANCEL procedure

The NLM CANCEL procedure allows clients to cancel outstanding
blocked lock requests, completing the set of lock-related
operations that share common lookup patterns. This patch
continues the xdrgen migration by converting the CANCEL
procedure, leveraging the same nlm4svc_lookup_host() and
nlm4svc_lookup_file() helpers established in the TEST procedure
conversion to maintain consistency across the series.

This patch converts the CANCEL procedure to use xdrgen functions
nlm4_svc_decode_nlm4_cancargs and nlm4_svc_encode_nlm4_res
generated from the NLM version 4 protocol specification. The
procedure handler uses xdrgen types through a wrapper structure
that bridges between generated code and the legacy nlm_lock
representation still used by the core lockd logic.

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.

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