lockd: Convert server-side undefined procedures to xdrgen

lockd: Convert server-side undefined procedures to xdrgen

The NLMv4 protocol defines several procedure slots that are
not implemented. These undefined procedures need proper
handling to return rpc_proc_unavail to clients that
mistakenly invoke them.

This patch converts the three undefined procedure entries
(slots 17, 18, and 19) to use xdrgen functions
nlm4_svc_decode_void and nlm4_svc_encode_void. The
nlm4svc_proc_unused function is also moved earlier in the
file to follow the convention of placing procedure
implementations before the procedure table.

The pc_argsize, pc_ressize, and pc_argzero fields are now
correctly set to zero since no arguments or results are
processed. The pc_xdrressize field is updated to XDR_void
to accurately reflect the response size.

This conversion completes the migration of all NLMv4
server-side procedures to use xdrgen-generated XDR
functions, improving type safety and eliminating
hand-written XDR code.

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