cgroup: reorder operations in cgroup_mkdir()

Currently, operations to initialize internal objects and create
interface directory and files are intermixed in cgroup_mkdir().  We're
in the process of refactoring cgroup and css management paths to
separate them out to eventually allow cgroups which aren't visible
through cgroup fs.

This patch reorders operations inside cgroup_mkdir() so that interface
directory and file handling comes after internal object
initialization.  This will enable further refactoring.

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