cgroup: use cached local variable parent in for loop

cgroup: use cached local variable parent in for loop

Use local variable parent to initialize iter tcgrp in for loop so the size
of cgroup.o can be reduced by 64 bytes. No functional change intended.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>