cgroup: clean up css_[try]get() and css_put()

* __css_get() isn't used by anyone.  Fold it into css_get().

* Add proper function comments to all css reference functions.

This patch is purely cosmetic.

v2: Typo fix as per Li.

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