NFSD: introduce netlink stubs

Generate stubs and uAPI for nfsd netlink protocol. For the moment,
the new protocol has one operation: rpc_status.

The generated header and source files are created by running:

tools/net/ynl/ynl-regen.sh

Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>