SUNRPC: Add a trace point in svc_xprt_deferred_close

The trace point in svc_xprt_close() reports only some local close
requests. Try to capture more local close requests.

Note that "trace-cmd record -T -e sunrpc:svc_xprt_close" will
neatly capture the identity of the caller requesting the close.

Signed-off-by: Chuck Lever <[e-mail]>

net/sunrpc/svc_xprt.c

···
  */
 void svc_xprt_deferred_close(struct svc_xprt *xprt)
 {
+	trace_svc_xprt_close(xprt);
 	if (!test_and_set_bit(XPT_CLOSE, &xprt->xpt_flags))
 		svc_xprt_enqueue(xprt);
 }