sysctl: add proc interface to set page cache limit

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


CVE: NA

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Add two proc interfaces to set page cache limit. Both
vm_cache_limit_mbytes and vm_cache_limit_ratio will
be update when writing either of the two interfaces.

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