mm/memcontrol: fix wrong vmstats for dying memcg

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5E8LA


CVE: NA

--------------------------------

At present, only when the absolute value of stat_cpu->count exceeds
MEMCG_CHARGE_BATCH will it be updated to stat, so there will always
be a certain lag difference between stat and the correct value.

In addition, since the partially deleted memcg is still referenced, it
will not be freed immediately after it is offline. Although the
remaining memcg has released the page, it and the parent's stat will
still be not 0 or too large due to the update lag, which leads to the
abnormality of the total_<count> parameter in the memory.stat file.

This patch mainly solves the problem of synchronization between
memcg's stat and the correct value during the destruction process
from two aspects:
1) Perform a flush synchronization operation when memcg is offline
2) For memcg in the process of being destroyed, bypass the threshold
   judgment when updating vmstats

Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>