blkcg: small fix on comment in blkcg_init_queue

The comment before blkg_create() in blkcg_init_queue() was moved
from blkcg_activate_policy() by commit ec13b1d6, but
it does not suit for the new context.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
CC: Tejun Heo <tj@kernel.org>
CC: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>