cgroup: relocate cgroup_populate_dir()

Move it upwards so that it's right below cgroup_clear_dir() and the
forward declaration is unnecessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>