cgroup: build list of all cgroups under a given cgroupfs_root

Build a list of all cgroups anchored at cgroupfs_root->allcg_list and
going through cgroup->allcg_node.  The list is protected by
cgroup_mutex and will be used to improve cgroup file handling.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>

diff --cc include/linux/cgroup.h
Simple merge

diff --cc kernel/cgroup.c
Simple merge