blk-flush: reuse rq queuelist in flush state machine

Since we don't need to maintain inflight flush_data requests list
anymore, we can reuse rq->queuelist for flush pending list.

Note in mq_flush_data_end_io(), we need to re-initialize rq->queuelist
before reusing it in the state machine when end, since the rq->rq_next
also reuse it, may have corrupted rq->queuelist by the driver.

This patch decrease the size of struct request by 16 bytes.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230717040058.3993930-5-chengming.zhou@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -178,7 +178,6 @@ struct request {
 
 	struct {
 		unsigned int		seq;
-		struct list_head	list;
 		rq_end_io_fn		*saved_end_io;
 	} flush;