[SCSI] fix qemu boot hang problem

The latest kernel fails to boot qemu arm images when using scsi
for disk access. Boot gets stuck after the following messages.

brd: module loaded
sym53c8xx 0000:00:0c.0: enabling device (0100 -> 0103)
sym0: <895a> rev 0x0 at pci 0000:00:0c.0 irq 93
sym0: No NVRAM, ID 7, Fast-40, LVD, parity checking
sym0: SCSI BUS has been reset.
scsi host0: sym-2.2.3

Bisect points to commit 71e75c9 ("scsi: convert device_busy to
atomic_t"). Code inspection shows the following suspicious change
in scsi_request_fn.

out_delay:
-       if (sdev->device_busy == 0 && !scsi_device_blocked(sdev))
+       if (atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
		blk_delay_queue(q, SCSI_QUEUE_DELAY);
	}

'sdev->device_busy == 0' was replaced with 'atomic_read(&sdev->device_busy)',
meaning the logic was reversed. Changing this expression to
'!atomic_read(&sdev->device_busy)' fixes the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: 71e75c9
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_lib.c

@@ -1774,7 +1774,7 @@ static void scsi_request_fn(struct request_queue *q)
 	blk_requeue_request(q, req);
 	atomic_dec(&sdev->device_busy);
 out_delay:
-	if (atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
+	if (!atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
 		blk_delay_queue(q, SCSI_QUEUE_DELAY);
 }