SUNRPC: Set rq_auth_stat in the pg_authenticate() callout

In a few moments, rq_auth_stat will need to be explicitly set to
rpc_auth_ok before execution gets to the dispatcher.

svc_authenticate() already sets it, but it often gets reset to
rpc_autherr_badcred right after that call, even when authentication
is successful. Let's ensure that the pg_authenticate callout and
svc_set_client() set it properly in every case.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>