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

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


CVE: NA

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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>