block: move initialization of q->blkg_list into blkcg_init_queue

q->blkg_list is only used by blkcg code, so move it into
blkcg_init_queue.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220211101149.2368042-3-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>