proc/meminfo: add "FileCache" item in /proc/meminfo

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


CVE: NA

--------------------------------
Item "FileCache" in /proc/meminfo show the number of page cache
in LRU(active + inactive).

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