cgroups: don't take into account a leading / for the "/" parent

Without this patch, we dump somethin like this:
{
	cnames: "hugetlb"
	dirs: 	{
		dir_name: ""
		children: 		{
			dir_name: "ewroot"
			children: <empty>
			properties: <empty>
		}

		properties: <empty>
	}

}

It's obvious, that dir_name should be newroot.

The problem is reproduced, if a task leaves in "/" and has a subgroup.
This issue was caught by a chance. The cgroup02 test doesn't clean up
controllers and leaves the "newroot" there. So when we executed a cgroup
test after cgroup02, we could find many directories like "ewroot",
"wroot", etc. This patch fixes this issue.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>