block: don't reorder requests in blk_mq_add_to_batch

LIFO ordering for batched completions is a bit unexpected and also
defeats some merging optimizations in e.g. the XFS buffered write
code.  Now that we can easily add the request to the tail of the list
do that.

Signed-off-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

include/linux/blk-mq.h

@@ -884,7 +884,7 @@ static inline bool blk_mq_add_to_batch(struct request *req,
 	else if (iob->complete != complete)
 		return false;
 	iob->need_ts |= blk_mq_need_time_stamp(req);
-	rq_list_add_head(&iob->req_list, req);
+	rq_list_add_tail(&iob->req_list, req);
 	return true;
 }