cgroup: drop hierarchy_id_lock

cgroup: drop hierarchy_id_lock

Now that hierarchy_id alloc / free are protected by the cgroup
mutexes, there's no need for this separate lock.  Drop it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Change-Id: I03fbc8bba08a785c6082a9b5bb1087c53c506c60
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=54e7b4eb15fc4354d5ada5469e3db4a220ddb3ed
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>