page_cgroup: fix horrid swap accounting regression

Why is memcg's swap accounting so broken? Insane counts, wrong
ownership, unfreeable structures, which later get freed and then
accessed after free.

Turns out to be a tiny a little 3.3-rc1 regression in 9fb4b7cc0724
"page_cgroup: add helper function to get swap_cgroup": the helper
function (actually named lookup_swap_cgroup()) returns an address using
void* arithmetic, but the structure in question is a short.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Bob Liu <lliubbo@gmail.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_cgroup.c

@@ -379,13 +379,15 @@ static struct swap_cgroup *lookup_swap_cgroup(swp_entry_t ent,
 	pgoff_t offset = swp_offset(ent);
 	struct swap_cgroup_ctrl *ctrl;
 	struct page *mappage;
+	struct swap_cgroup *sc;
 
 	ctrl = &swap_cgroup_ctrl[swp_type(ent)];
 	if (ctrlp)
 		*ctrlp = ctrl;
 
 	mappage = ctrl->map[offset / SC_PER_PAGE];
-	return page_address(mappage) + offset % SC_PER_PAGE;
+	sc = page_address(mappage);
+	return sc + offset % SC_PER_PAGE;
 }
 
 /**