This finally kills of blk_queue_empty().  This is similar to the patch I
recently sent to fix the SCSI logic as well.  A lot of drivers are doing
this in our core, mainly because that is the way they always did it:

start_queue:
	if (blk_queue_empty(q))
		return;

	rq = elv_next_request(q);
	if (!rq)
		return;

Patch simply removes the blk_queue_empty() check, and adds a check for
!rq return from elv_next_request() if the driver didn't already do that.

Additionally, the AS io scheduler can return NULL from
elv_next_request() if it thinks this is best. This way we are also
prepared for that to work well.

Patch was done by Nick Piggin.