cgroup: rename cgroup->dummy_css to ->self and move it to the top

cgroup->dummy_css is used as the placeholder css when performing css
oriended operations on the cgroup.  We're gonna shift more cgroup
management to this css.  Let's rename it to ->self and move it to the
top.

This is pure rename and field relocation.

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