SUNRPC: Merge svc_do_enqueue_xprt() into svc_enqueue_xprt()

Neil says:
"These functions were separated in commit 0971374e ("SUNRPC:
Reduce contention in svc_xprt_enqueue()") so that the XPT_BUSY check
happened before taking any spinlocks.

We have since moved or removed the spinlocks so the extra test is
fairly pointless."

I've made this a separate patch in case the XPT_BUSY change has
unexpected consequences and needs to be reverted.

Suggested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>