blk-cgroup: refactor blkcg_print_stat

Factor out a helper to deal with a single blkcg_gq to make the code a
little bit easier to follow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210810152623.1796144-1-hch@lst.de


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