Return ErrCgroupDeleted when no subsystems
Return ErrCgroupDeleted when no subsystems

Return the error when 0 subsystems are active signaling that the cgroup
has been deleted.

Fix for: https://github.com/containerd/containerd/issues/3133

Signed-off-by: Michael Crosby <951f4c7b5d47e32d0112c78d14a98eaa23e6b5ef@gmail.com>

containerd/cgroups