SUNRPC: Update debugging instrumentation in xprt_do_reserve()

Replace a dprintk() with a tracepoint. The tracepoint marks the
point where an RPC request is assigned an XID.

Additional clean up: Remove trace_xprt_enq_xmit, which reports much
the same thing. That tracepoint was added for debugging commit
918f3c1f ("SUNRPC: Improve latency for interactive tasks").

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