Partition include/linux/blk-cgroup.h into two parts: one is public part,
the other is block layer private part.

Suggested by Christoph Hellwig.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220211101149.2368042-4-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>