lockd: Use xdrgen XDR functions for the NLMv3 LOCK_RES procedure

lockd: Use xdrgen XDR functions for the NLMv3 LOCK_RES procedure

Continue the xdrgen migration by converting NLMv3 LOCK_RES,
the callback that a remote NLM uses to return async LOCK
results to this lockd.  The procedure now uses
nlm_svc_decode_nlm_res and nlm_svc_encode_void, generated
from the NLM version 3 protocol specification.

Setting pc_argzero to zero is safe because the generated
decoder fills the argp->xdrgen subfields before the procedure
runs, so the zeroing memset performed by the dispatch layer
is no longer needed.

Setting pc_xdrressize to XDR_void reflects that LOCK_RES, as
a callback, returns no data; the previous value of St
over-reserved a status word in the reply buffer.

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