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>
diff --git a/block/blk-core.c b/block/blk-core.c
index 4ce953f..fc7d8ad 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -136,7 +136,7 @@
 {
 	struct request_queue *q = rq->q;
 
-	if (&q->flush_rq != rq) {
+	if (!(rq->cmd_flags & REQ_FLUSH_SEQ)) {
 		if (error)
 			clear_bit(BIO_UPTODATE, &bio->bi_flags);
 		else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
@@ -1789,7 +1789,7 @@
 	 * normal IO on queueing nor completion.  Accounting the
 	 * containing request is enough.
 	 */
-	if (blk_do_io_stat(req) && req != &req->q->flush_rq) {
+	if (blk_do_io_stat(req) && !(req->cmd_flags & REQ_FLUSH_SEQ)) {
 		unsigned long duration = jiffies - req->start_time;
 		const int rw = rq_data_dir(req);
 		struct hd_struct *part;