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>