blk-mq: don't use the requeue list to queue flush commands

Currently both requeues of commands that were already sent to the driver
and flush commands submitted from the flush state machine share the same
requeue_list struct request_queue, despite requeues doing head
insertions and flushes not.  Switch to using two separate lists instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230519044050.107790-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -487,6 +487,7 @@ struct request_queue {
 	 * for flush operations
 	 */
 	struct blk_flush_queue	*fq;
+	struct list_head	flush_list;
 
 	struct list_head	requeue_list;
 	spinlock_t		requeue_lock;