lockd: Use xdrgen XDR functions for the NLMv4 TEST procedure

lockd: Use xdrgen XDR functions for the NLMv4 TEST procedure

The NLM TEST procedure requires host and file lookups to check
lock state, operations that will be common across multiple NLM
procedures being migrated to xdrgen. By introducing the helper
functions nlm4svc_lookup_host() and nlm4svc_lookup_file() now,
we establish reusable patterns for subsequent conversions in
this series.

This patch converts the TEST procedure to use xdrgen functions
nlm4_svc_decode_testargs and nlm4_svc_encode_testres generated
from the NLM version 4 protocol specification. The procedure
handler is rewritten to use xdrgen types through wrapper
structures that bridge between generated code and the legacy
nlm_lock representation still used by the core lockd logic.
TEST_MSG is to be converted in a subsequent patch.

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>