lockd: Use xdrgen XDR functions for the NLMv4 NULL procedure

lockd: Use xdrgen XDR functions for the NLMv4 NULL procedure

Hand-written XDR encoders and decoders are difficult to maintain
and can inadvertently diverge from protocol specifications. By
migrating to xdrgen-generated code, we improve type safety and
ensure the implementation exactly matches the NLM version 4
protocol specification.

This patch begins the migration by converting the NULL procedure
to use nlm4_svc_decode_void and nlm4_svc_encode_void generated
from Documentation/sunrpc/xdr/nlm4.x. The NULL procedure is
straightforward as it has no arguments or results, making it an
ideal starting point for this series.

The pc_xdrressize field is set to XDR_void (zero) to reflect
that this procedure returns no XDR-encoded data. The argzero
field is also set to zero since xdrgen decoders reliably
initialize all decoded values.

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