cgroup: add css_set->dfl_cgrp

To implement the unified hierarchy behavior, we'll need to be able to
determine the associated cgroup on the default hierarchy from css_set.
Let's add css_set->dfl_cgrp so that it can be accessed conveniently
and efficiently.

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