mm:vmscan: add the missing check of page_cache_over_limit

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4SK3S


CVE: NA

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Function shrink_shepherd is used to queue each work on cpu to shrink
page cache, and it will be called periodically, but if there is no
page_cache_over_limit check before shrink page cache, it will result
in periodic memory reclamation even the number of page cache below
limit, so add basic check before shrink page cache.

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>