mm: memcontrol: clean up memcg zoneinfo lookup

mm: memcontrol: clean up memcg zoneinfo lookup

Memcg zoneinfo lookup sites have either the page, the zone, or the node id
and zone index, but sites that only have the zone have to look up the node
id and zone index themselves, whereas sites that already have those two
integers use a function for a simple pointer chase.

Provide mem_cgroup_zone_zoneinfo() that takes a zone pointer and let sites
that already have node id and zone index - all for each node, for each
zone iterators - use &memcg->nodeinfo[nid]->zoneinfo[zid].

Rename page_cgroup_zoneinfo() to mem_cgroup_page_zoneinfo() to match.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>