ide: dequeue in-flight request

ide generally has single request in flight and tracks it using
hwif->rq and all state handlers follow the following convention.

* ide_started is returned if the request is in flight.

* ide_stopped is returned if the queue needs to be restarted.  The
  request might or might not have been processed fully or partially.

* hwif->rq is set to NULL, when an issued request completes.

So, dequeueing model can be implemented by dequeueing after fetch,
requeueing if hwif->rq isn't NULL on ide_stopped return and doing
about the same thing on completion / port unlock paths.  These changes
can be made in ide-io proper.

In addition to the above main changes, the following updates are
necessary.

* ide-cd shouldn't dequeue a request when issuing REQUEST SENSE for it
  as the request is already dequeued.

* ide-atapi uses request queue as stack when issuing REQUEST SENSE to
  put the REQUEST SENSE in front of the failed request.  This now
  needs to be done using requeueing.

[ Impact: dequeue in-flight request ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Borislav Petkov <petkovbb@googlemail.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/ide/ide-cd.c

@@ -405,15 +405,7 @@ static int cdrom_decode_status(ide_drive_t *drive, u8 stat)
 
 end_request:
 	if (stat & ATA_ERR) {
-		struct request_queue *q = drive->queue;
-		unsigned long flags;
-
-		spin_lock_irqsave(q->queue_lock, flags);
-		blkdev_dequeue_request(rq);
-		spin_unlock_irqrestore(q->queue_lock, flags);
-
 		hwif->rq = NULL;
-
 		return ide_queue_sense_rq(drive, rq) ? 2 : 1;
 	} else
 		return 2;