cgroup: initialize cgrp->allcg_node in init_cgroup_housekeeping()

Not strictly necessary but it's annoying to have uninitialized
list_head around.

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

diff --cc kernel/cgroup.c
Simple merge