cgroup: make hierarchy_id use cyclic idr

cgroup: make hierarchy_id use cyclic idr

We want to be able to lookup a hierarchy from its id and cyclic
allocation is a whole lot simpler with idr.  Convert to idr and use
idr_alloc_cyclc().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Change-Id: Ibd20ebe71ddb452178302cf86a22572b18cd82df
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=1a574231669f8c3065c83974e9557fcbbd94b8a6
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>