blk-throtl: stop using blkg->stat_bytes and ->stat_ios

When used on cgroup1, blk-throtl uses the blkg->stat_bytes and
->stat_ios from blk-cgroup core to populate four stat knobs.
blk-cgroup core is moving away from blkg_rwstat to improve scalability
and won't be able to support this usage.

It isn't like the sharing gains all that much.  Let's break them out
to dedicated rwstat counters which are updated when on cgroup1.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>