From: Jens Axboe <axboe@suse.de>

There's a small discrepancy in when we decide to unplug a queue based on
q->unplug_thresh.  Basically it doesn't work for tagged queues, since
q->rq.count[READ] + q->rq.count[WRITE] is just the number of allocated
requests, not the number of requests stuck in the io scheduler.  We could
just change the nr_queued == to a nr_queued >=, however that is still
suboptimal.

This patch adds accounting for requests that have been dequeued from the io
scheduler, but not freed yet.  These are q->in_flight.  allocated_requests
- q->in_flight == requests_in_scheduler.  So the condition correctly
becomes

	if (requests_in_scheduler == q->unplug_thresh)

instead.  I did a quick round of testing, and for dbench on a SCSI disk the
number of timer induced unplugs was reduced from 13 to 5 :-).  Not a huge
number, but there might be cases where it's more significant.  Either way,
it gets ->unplug_thresh always right, which the old logic didn't.