cgfsng: try to delete parent cgroups

cgfsng: try to delete parent cgroups

Say we have

    lxc.uts.name = c1
    lxc.cgroup.dir = lxd/a/b/c

the path for the container's cgroup would be

    lxd/a/b/c/c1

When the container is shutdown we should not just try to delete "c1" we should
also try to delete "c", "b", "a", and "lxd". This is to ensure that we don't
leave empty cgroups around thereby increasing the chance that we run into
trouble with cgroup limits. The algorithm for this isn't too costly since we
can simply stop walking upwards at the first rmdir() failure.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>