blkcg: put back rcu lock in blkcg_bio_issue_check()

I was a little overzealous in removing the rcu_read_lock() call from
blkcg_bio_issue_check() and it broke blk-throttle. Put it back.

Fixes: e35403a034bf ("blkcg: associate blkg when associating a device")
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-cgroup.h

@@ -764,6 +764,8 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	struct blkcg_gq *blkg;
 	bool throtl = false;
 
+	rcu_read_lock();
+
 	if (!bio->bi_blkg) {
 		char b[BDEVNAME_SIZE];
 
@@ -791,6 +793,7 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 
 	blkcg_bio_issue_init(bio);
 
+	rcu_read_unlock();
 	return !throtl;
 }