block: Use pointer to backing_dev_info from request_queue

We will want to have struct backing_dev_info allocated separately from
struct request_queue. As the first step add pointer to backing_dev_info
to request_queue and convert all users touching it. No functional
changes in this patch.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/md/dm-thin.c

@@ -2711,7 +2711,7 @@ static int pool_is_congested(struct dm_target_callbacks *cb, int bdi_bits)
 		return 1;
 
 	q = bdev_get_queue(pt->data_dev->bdev);
-	return bdi_congested(&q->backing_dev_info, bdi_bits);
+	return bdi_congested(q->backing_dev_info, bdi_bits);
 }
 
 static void requeue_bios(struct pool *pool)