sunrpc: Move trace_svc_xprt_dequeue()

Reduce the amount of noise generated by trace_svc_xprt_dequeue by
moving it to the end of svc_get_next_xprt. This generates exactly
one trace event when a ready xprt is found, rather than spurious
events when there is no work to do. The empty events contain no
information that can't be obtained simply by tracing function calls
to svc_xprt_dequeue.

A small additional benefit is simplification of the svc_xprt_event
trace class, which no longer has to handle the case when the @xprt
parameter is NULL.

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