[RFC][PATCH 1/4] memcg: simplify page cache charging

[KAMEZAWA Hiroyuki]

This is an add-on patches to mem_cgroup_replace_page_cache(), introducing
new LRU rule under memcg, finally. After this, lru handling will be
much simplified. (all pathces are based on 3.2.0-rc4-next-20111205+)

But this is experimental... I may forget some important corner cases.
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