SUNRPC: rename and refactor svc_get_next_xprt()

svc_get_next_xprt() does a lot more than just get an xprt.  It also
decides if it needs to sleep, depending not only on the availability of
xprts but also on the need to exit or handle external work.

So rename it to svc_rqst_wait_for_work() and only do the testing and
waiting.  Move all the waiting-related code out of svc_recv() into the
new svc_rqst_wait_for_work().

Move the dequeueing code out of svc_get_next_xprt() into svc_recv().

Previously svc_xprt_dequeue() would be called twice, once before waiting
and possibly once after.  Now instead rqst_should_sleep() is called
twice.  Once to decide if waiting is needed, and once to check against
after setting the task state do see if we might have missed a wakeup.

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