cgroup: always use cgroup_next_child() to walk the children list

There are several places where the children list is accessed directly.
This patch converts those places to use cgroup_next_child().  This
will help updating the hierarchy iterators to use @css instead of
@cgrp.

While cgroup_next_child() can be heavy in pathological cases - e.g. a
lot of dead children, this shouldn't cause any noticeable behavior
differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>