mm/page_cache_limit: reconfiguration about page cache limit when memory plug/unplug

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4HOXK



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kthread and page cache limit should be reconfigured when memory
hot plug and hot unplug.

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>