cgroup: remove cftype_set

cftype_set was added primarily to allow registering the same cftype
array more than once for different subsystems.  Nobody uses or needs
such thing and it's already broken because each cftype has ->ss
pointer which is initialized during registration.

Let's add list_head ->node to cftype and use the first cftype entry in
the array to link them instead of allocating separate cftype_set.
While at it, trigger WARN if cft seems previously initialized during
registration.

This simplifies cftype handling a bit.

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