cgroup: implement CFTYPE_ONLY_ON_DFL

This cftype flag makes the file only appear on the default hierarchy.
This will later be used for cgroup.controllers file.

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

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

diff --cc kernel/cgroup.c
Simple merge