SUNRPC: Make the svc_authenticate tracepoint conditional

Clean up: Simplify the tracepoint's only call site.

Also, I noticed that when svc_authenticate() returns SVC_COMPLETE,
it leaves rq_auth_stat set to an error value. That doesn't need to
be recorded in the trace log.

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