cgroup: move cgroup->sibling and ->children into cgroup_subsys_state

We're moving towards using cgroup_subsys_states as the fundamental
structural blocks.  Let's move cgroup->sibling and ->children into
cgroup_subsys_state.  This is pure move without functional change and
only cgroup->self's fields are actually used.  Other csses will make
use of the fields later.

While at it, update init_and_link_css() so that it zeroes the whole
css before initializing it and remove explicit zeroing of ->flags.

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