memcg: factor out mem_cgroup_stat_aggregate()

Currently mem_cgroup_css_rstat_flush() is used to flush the per-CPU
statistics from a specified CPU into the global statistics of the
memcg. It processes three kinds of data in three for loops using exactly
the same method. Therefore, the for loop can be factored out and may
make the code more clean.

Link: https://lkml.kernel.org/r/20241026093407.310955-1-xiujianfeng@huaweicloud.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Wang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>