[S390] tape: Fix race condition in tape block device driver

Due to incorrect function call sequence it can happen that a tape block
request is finished before the request is taken from the block request queue.

The following sequence leads to that condition:
 * tapeblock_start_request() -> start CCW program
 * Request finishes -> IO interrupt
 * tapeblock_end_request()
 * end_that_request_last()

If blkdev_dequeue_request() has not been called before end_that_request_last(),
a kernel bug is triggered in end_that_request_last() because the request is
still queued. To solve that problem blkdev_dequeue_request() has to be called
before starting the CCW program.
Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/tape_block.c

@@ -179,11 +179,11 @@ tapeblock_requeue(struct work_struct *work) {
 			tapeblock_end_request(req, -EIO);
 			continue;
 		}
+		blkdev_dequeue_request(req);
+		nr_queued++;
 		spin_unlock_irq(&device->blk_data.request_queue_lock);
 		rc = tapeblock_start_request(device, req);
 		spin_lock_irq(&device->blk_data.request_queue_lock);
-		blkdev_dequeue_request(req);
-		nr_queued++;
 	}
 	spin_unlock_irq(&device->blk_data.request_queue_lock);
 	atomic_set(&device->blk_data.requeue_scheduled, 0);