memcg: further simplify mem_cgroup_iter

memcg: further simplify mem_cgroup_iter

mem_cgroup_iter basically does two things currently.  It takes care of the
house keeping (reference counting, raclaim cookie) and it iterates through
a hierarchy tree (by using cgroup generic tree walk).  The code would be
much more easier to follow if we move the iteration outside of the
function (to __mem_cgrou_iter_next) so the distinction is more clear.
This patch doesn't introduce any functional changes.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Ying Han <yinghan@google.com>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>