memcg: save 20% of per-page memcg memory overhead

This patch series removes the direct page pointer from struct
page_cgroup, which saves 20% of per-page memcg memory overhead (Fedora
and Ubuntu enable memcg per default, openSUSE apparently too).

The node id or section number is encoded in the remaining free bits of
pc->flags which allows calculating the corresponding page without the
extra pointer.

I ran, what I think is, a worst-case microbenchmark that just cats a
large sparse file to /dev/null, because it means that walking the LRU
list on behalf of per-cgroup reclaim and looking up pages from
page_cgroups is happening constantly and at a high rate.  But it made
no measurable difference.  A profile reported a 0.11% share of the new
lookup_cgroup_page() function in this benchmark.

	Hannes
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