block: use proper BLK_RW_ASYNC in blk_queue_start_tag()

Makes it easier to read than the 0.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/blk-tag.c b/block/blk-tag.c
index 2e5cfeb..6b0f52c 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -359,7 +359,7 @@
 		max_depth -= 2;
 		if (!max_depth)
 			max_depth = 1;
-		if (q->in_flight[0] > max_depth)
+		if (q->in_flight[BLK_RW_ASYNC] > max_depth)
 			return 1;
 	}