cgroup: relocate functions in preparation of kernfs conversion

Relocate cgroup_init/exit_root_id(), cgroup_free_root(),
cgroup_kill_sb() and cgroup_file_name() in preparation of kernfs
conversion.

These are pure relocations to make kernfs conversion easier to follow.

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