memcg: add drop_caches interface

We drop caches by traversing memcg's local lru to invalidate pages. Now it
only supports drop file caches, since slab is not accounted in memcg under
this version.

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
Merge pull request #43 from shazj99/memcg_drop_cache

memcg: add drop_caches interface