blkcg: fix incorrect read/write sync/async stat accounting

While unifying how blkcg stats are collected, 77ea7338 ("blkcg:
move io_service_bytes and io_serviced stats into blkcg_gq")
incorrectly used bio->flags instead of bio->rw to tell the IO type.
This made IOs to be accounted as the wrong type.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 77ea7338 ("blkcg: move io_service_bytes and io_serviced stats into blkcg_gq")
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-cgroup.h

@@ -713,9 +713,9 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 
 	if (!throtl) {
 		blkg = blkg ?: q->root_blkg;
-		blkg_rwstat_add(&blkg->stat_bytes, bio->bi_flags,
+		blkg_rwstat_add(&blkg->stat_bytes, bio->bi_rw,
 				bio->bi_iter.bi_size);
-		blkg_rwstat_add(&blkg->stat_ios, bio->bi_flags, 1);
+		blkg_rwstat_add(&blkg->stat_ios, bio->bi_rw, 1);
 	}
 
 	rcu_read_unlock();