blk-iocost: combine local_stat and desc_stat to stat

When we flush usage, wait, indebt stat in iocg_flush_stat(), we use
local_stat and desc_stat, which has no point since the leaf iocg
only has local_stat and the inner iocg only has desc_stat. Also
we don't need to flush percpu abs_vusage for these inner iocgs.

This patch combine local_stat and desc_stat to stat, only flush
percpu abs_vusage for active leaf iocgs, then build inner walk
list to propagate.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220510034757.21761-1-zhouchengming@bytedance.com


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