cgroup/cgroup.c: replace 'of->kn->priv' with of_cft()

we have supplied the inline function: of_cft() in cgroup.h.

So replace the direct use 'of->kn->priv' with inline func
of_cft(), which is more readable.

Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Tejun Heo <tj@kernel.org>