SUNRPC: Move trace_svc_xprt_enqueue

The xpt_flags field frequently changes between the time that
svc_xprt_ready() grabs a copy and execution flow arrives at the
tracepoint at the tail of svc_xprt_enqueue(). In fact, there's
usually a sleep/wake-up in there, so those flags are almost
guaranteed to be different.

It would be more useful to record the exact flags that were used to
decide whether the transport is ready, so move the tracepoint.

Moving it means the tracepoint can't pick up the waker's pid. That
can be added to struct svc_rqst if it turns out that is important.

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