cgroup: trivial cleanup for cgroup_init/load_subsys()

Consistently use @css and @dummytop in these two functions instead of
referring to them indirectly.

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

diff --cc kernel/cgroup.c
Simple merge