block: add REQ_FLUSH_SEQ

rq == &q->flush_rq was used to determine whether a rq is part of a
flush sequence, which worked because all requests in a flush sequence
were sequenced using the single dedicated request.  This is about to
change, so introduce REQ_FLUSH_SEQ flag to distinguish flush sequence
requests.

This patch doesn't cause any behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-flush.c

@@ -130,6 +130,7 @@ static struct request *queue_next_fseq(struct request_queue *q)
 		BUG();
 	}
 
+	rq->cmd_flags |= REQ_FLUSH_SEQ;
 	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 	return rq;
 }