cgroup: convert cgroup_next_sibling() to cgroup_next_child()

cgroup is transitioning to using css (cgroup_subsys_state) as the main
subsys interface handle instead of cgroup and the iterators will be
updated to use css too.  The iterators need to walk the cgroup
hierarchy and return the css's matching the origin css, which is a bit
cumbersome to open code.

This patch converts cgroup_next_sibling() to cgroup_next_child() so
that it can handle all steps of direct child iteration.  This will be
used to update iterators to take @css instead of @cgrp.  In addition
to the new iteration init handling, cgroup_next_child() is
restructured so that the different branches share the end of iteration
condition check.

This patch doesn't change any behavior.

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