page_cgroup: cleanup lookup_swap_cgroup()

page_cgroup: cleanup lookup_swap_cgroup()

This patch is based on my previous patch:
page_cgroup: add helper function to get swap_cgroup

As Johannes suggested, change the public interface to
lookup_swap_cgroup_id(), replace swap_cgroup_getsc() with
lookup_swap_cgroup() and do some extra cleanup.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>