cgroup2: Manager.Destroy() should recursively delete subgroups

[AkihiroSuda]

$ docker version
Client:
 Version:           20.03.0-dev
 API version:       1.41
 Go version:        go1.13.10
 Git commit:        5bf109154
 Built:             Thu May 14 05:59:55 2020
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server:
 Engine:
  Version:          dev
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.10
  Git commit:       4e102ab1f0
  Built:            Thu May 14 05:57:27 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          v1.3.0-577-g7207226e
  GitCommit:        7207226e9d9aa6af9b48b681137139873b7c692d
 runc:
  Version:          1.0.0-rc10+dev
  GitCommit:        3f1e88699199d844dd211cbf5108eeff9444674e
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

$ docker run -it --rm --privileged alpine mkdir /sys/fs/cgroup/foo
$ ls /sys/fs/cgroup/docker/
7643812e0a0390cc595543190844ab9c10e064646b563549c17e3c05ce519523  cpu.pressure           io.pressure          memory.stat
cgroup.controllers                                                cpuset.cpus            io.stat              memory.swap.current
cgroup.events                                                     cpuset.cpus.effective  io.weight            memory.swap.events
cgroup.freeze                                                     cpuset.cpus.partition  memory.current       memory.swap.max
cgroup.max.depth                                                  cpuset.mems            memory.events        pids.current
cgroup.max.descendants                                            cpuset.mems.effective  memory.events.local  pids.events
cgroup.procs                                                      cpu.stat               memory.high          pids.max
cgroup.stat                                                       cpu.uclamp.max         memory.low           rdma.current
cgroup.subtree_control                                            cpu.uclamp.min         memory.max           rdma.max
cgroup.threads                                                    cpu.weight             memory.min
cgroup.type                                                       cpu.weight.nice        memory.oom.group
cpu.max                                                           io.max                 memory.pressure
$ ls /sys/fs/cgroup/docker/7643812e0a0390cc595543190844ab9c10e064646b563549c17e3c05ce519523/
cgroup.controllers      cgroup.threads         cpuset.mems.effective  io.pressure          memory.max           pids.current
cgroup.events           cgroup.type            cpu.stat               io.stat              memory.min           pids.events
cgroup.freeze           cpu.max                cpu.uclamp.max         io.weight            memory.oom.group     pids.max
cgroup.max.depth        cpu.pressure           cpu.uclamp.min         memory.current       memory.pressure      rdma.current
cgroup.max.descendants  cpuset.cpus            cpu.weight             memory.events        memory.stat          rdma.max
cgroup.procs            cpuset.cpus.effective  cpu.weight.nice        memory.events.local  memory.swap.current
cgroup.stat             cpuset.cpus.partition  foo                    memory.high          memory.swap.events
cgroup.subtree_control  cpuset.mems            io.max                 memory.low           memory.swap.max

Manager.Destroy() should recursively delete subgroups.