mm/page_cache_limit: add pagecache limit proc interface

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



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Add 3 proc interface to control page cache limit behavior:
1. switch of the this feature
2. control the page cache limit
3. control the ratio to reclaim when page cache beyond page cache limit

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