runsc: check for nested cgroup when generating croup paths

in nested container, we see paths from host in /proc/self/cgroup, so we
need to re-process that path to get a relative path to be used inside
the container.

Without it, runsc generates ugly paths that may trip other cgroup
watchers that expect clean paths. An example of ugly path is:

```
/sys/fs/cgroup/memory/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93/cgroupPath
```

Notice duplication of `docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93`

`/proc/1/cgroup` looks like

```
12:perf_event:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
11:blkio:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
10:freezer:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
9:hugetlb:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
8:devices:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
7:rdma:/
6:pids:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
5:cpuset:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
4:cpu,cpuacct:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
3:memory:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
2:net_cls,net_prio:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
1:name=systemd:/docker/e383892b29290ae8005d535f2dadc4a583bb354d5bb1ba8c10bf900d92c4db93
0::/system.slice/containerd.service
```

This is not necessary when the parent container was created with cgroup
namespace, but that setup is not very common right now.

Signed-off-by: Daniel Dao <dqminh89@gmail.com>

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