Remove leading / on cgroup v1

[haesbaert]

On cgroup v1 we get this:

  CGRP  cgroup=/
11:devices:/system.slice/systemd-journald.service
10:perf_event:/
9:cpu,cpuacct:/
8:blkio:/
7:memory:/system.slice/systemd-journald.service
6:hugetlb:/
5:cpuset:/
4:pids:/system.slice/systemd-journald.service
3:rdma:/
2:net_cls,net_prio:/
1:name=systemd:/system.slice/systemd-journald.service

This is likely because I only considered v2 in d3ca133