blkfront: Fix backtrace in del_gendisk

Commit 89de1669 upstream.

The call to del_gendisk follows an non-refcounted gd->queue
pointer. We release the last ref in blk_cleanup_queue. Fixed by
reordering releases accordingly.
Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/block/xen-blkfront.c

@@ -942,11 +942,11 @@ static void blkfront_closing(struct xenbus_device *dev)
 	/* Flush gnttab callback work. Must be done with no locks held. */
 	flush_scheduled_work();
 
+	del_gendisk(info->gd);
+
 	blk_cleanup_queue(info->rq);
 	info->rq = NULL;
 
-	del_gendisk(info->gd);
-
  out:
 	xenbus_frontend_closed(dev);
 }