cgroups: populate hierarchy for device cgroup

cgroups: populate hierarchy for device cgroup

With the changes introduced in:
b7b1e3a34ce28b01206c48227930ff83d399e7b6
the hierarchy-struct did not have the path_lim set anymore, which is
needed by setup_limits_legacy (->cg_legacy_set_data->lxc_write_openat)
to actually access the cgroup directory.

The issue can be reproduced with a container config having
```
lxc.cgroup.devices.deny = a
```
(or any lxc.cgroup.devices entry) set on a system booted with
systemd.unified_cgroup_hierarchy=0.

This affects all privileged containers on PVE (due to the default
devices.deny entry).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>