blk-cgroup: set blkg iostat after percpu stat aggregation

blk-cgroup: set blkg iostat after percpu stat aggregation

[ Upstream commit 258c53faaa228745cc6219c39c1a9a34d9652213 ]

Don't need to do blkg_iostat_set for top blkg iostat on each CPU,
so move it after percpu stat aggregation.

Fixes: 4d018b7c51aa ("blk-cgroup: show global disk stats in root cgroup io.stat")
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220213085902.88884-1-zhouchengming@bytedance.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>