blk-cgroup: factor out blkcg_free_all_cpd()

To reduce some duplicated code, factor out blkcg_free_all_cpd(). No
functional change.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220629070917.3113016-3-yanaijie@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>