virtio_blk: replace 0 by HCTX_TYPE_DEFAULT to index blk_mq_tag_set->map

Use HCTX_TYPE_DEFAULT instead of 0 to avoid hardcoding.
Signed-off-by: NDongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/virtio_blk.c

@@ -691,7 +691,8 @@ static int virtblk_map_queues(struct blk_mq_tag_set *set)
 {
 	struct virtio_blk *vblk = set->driver_data;
 
-	return blk_mq_virtio_map_queues(&set->map[0], vblk->vdev, 0);
+	return blk_mq_virtio_map_queues(&set->map[HCTX_TYPE_DEFAULT],
+					vblk->vdev, 0);
 }
 
 #ifdef CONFIG_VIRTIO_BLK_SCSI