lockd: Use xdrgen XDR functions for the NLMv4 TEST_RES procedure

lockd: Use xdrgen XDR functions for the NLMv4 TEST_RES procedure

Convert the TEST_RES procedure to use xdrgen functions
nlm4_svc_decode_nlm4_testres and nlm4_svc_encode_void.
TEST_RES is a callback procedure where the client sends
test lock results back to the server after an async TEST
request.

The pc_argzero field is set to zero because xdrgen decoders
reliably initialize all arguments, making the early
defensive memset unnecessary.

This change also corrects the pc_xdrressize field, which
previously contained a placeholder value.

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