blk-cgroup: remove open coded blkg_lookup instances

Use blkg_lookup instead of open coding it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Herrmann <aherrmann@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220921180501.1539876-4-hch@lst.de


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