memcg: support memcg sync reclaim work as kswapd

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4IMAK?from=project-issue


CVE: NA

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Since memory.high reclaim is sync whether is in interrupt, it could
do more work than direct reclaim, i.e. write out dirty page, etc.

So, add PF_KSWAPD flag, so that current_is_kswapd() would return true
for memcg kswapd.

Memcg kswapd should stop when usage of memcg fit the memcg kswapd stop
flag. When the userland sets the memcg->memory.max, the stop_flag is
(memcg->memory.high - memcg->memory.max * 10 / 1000), which is similar
with global kswapd. Otherwise, the stop_flag is (memcg->memory.high -
memcg->memory.high / 6), which is similar with most difference between
watermark_low and watermark_high.

And, memcg kswapd should not break memory.low protection for now.

Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: weiyang wang <wangweiyang2@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>