cgroup: reserve ID 0 for dummy_root and 1 for unified hierarchy

Before 1a574231 ("cgroup: make hierarchy_id use cyclic idr"),
hierarchy IDs were allocated from 0.  As the dummy hierarchy was
always the one first initialized, it got assigned 0 and all other
hierarchies from 1.  The patch accidentally changed the minimum
useable ID to 2.

Let's restore ID 0 for dummy_root and while at it reserve 1 for
unified hierarchy.

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