cgroup: remove NULL checks from [pr_cont_]cgroup_{name|path}()

The dummy hierarchy is now a fully functional one and dummy_top has a
kernfs_node associated with it.  Drop the NULL checks in
[pr_cont_]cont_{name|path}() which are no longer necessary.

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