blk-mq-sched: don't add flushes to the head of requeue queue

If we are currently out of driver tags, we don't want to add a
new flush (without a tag) to the head of the requeue list. We
want to add it to the back, behind the others that are
potentially also waiting for a tag.
Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>

block/blk-mq-sched.c

@@ -351,7 +351,7 @@ static void blk_mq_sched_insert_flush(struct blk_mq_hw_ctx *hctx,
 		blk_insert_flush(rq);
 		blk_mq_run_hw_queue(hctx, true);
 	} else
-		blk_mq_add_to_requeue_list(rq, true, true);
+		blk_mq_add_to_requeue_list(rq, false, true);
 }
 
 void blk_mq_sched_insert_request(struct request *rq, bool at_head,