blk-cgroup: cleanup blk_cgroup_congested

Use blkcg_css instead of open coding it, and switch to a slightly
more natural for loop.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220420042723.1010598-14-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>