memcg: css_put after remove_list

mem_cgroup_uncharge_page does css_put on the mem_cgroup before uncharging from
it, and before removing page_cgroup from one of its lru lists: isn't there a
danger that struct mem_cgroup memory could be freed and reused before
completing that, so corrupting something?  Never seen it, and for all I know
there may be other constraints which make it impossible; but let's be
defensive and reverse the ordering there.

mem_cgroup_force_empty_list is safe because there's an extra css_get around
all its works; but even so, change its ordering the same way round, to help
get in the habit of doing it like this.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Balbir Singh <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: Hirokazu Takahashi <[email protected]>
Cc: YAMAMOTO Takashi <[email protected]>
Cc: Paul Menage <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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