NLM: Fix svcxdr_encode_owner()

Dai Ngo reports that, since the XDR overhaul, the NLM server crashes
when the TEST procedure wants to return NLM_DENIED. There is a bug
in svcxdr_encode_owner() that none of our standard test cases found.

Replace the open-coded function with a call to an appropriate
pre-fabricated XDR helper.

Reported-by: Dai Ngo <Dai.Ngo@oracle.com>
Fixes: a6a63ca5652e ("lockd: Common NLM XDR helpers")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

lockd: Common NLM XDR helpers

Add a .h file containing xdr_stream-based XDR helpers common to both
NLMv3 and NLMv4.

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