lockd: Create a simplified .vs_dispatch method for NLM requests

[ Upstream commit a9ad1a80 ]

To enable xdr_stream-based encoding and decoding, create a bespoke
RPC dispatch function for the lockd service.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>