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>

cgroup.c

@@ -676,7 +676,9 @@ static int dump_cg_dirs(struct list_head *dirs, size_t n_dirs, CgroupDirEntry **
 
 	list_for_each_entry(cur, dirs, siblings) {
 		cgroup_dir_entry__init(cde);
-		cde->dir_name = cur->path + poff + 1 /* leading / */;
+		cde->dir_name = cur->path + poff;
+		if (poff != 1) /* parent isn't "/" */
+			cde->dir_name++; /* leading / */
 		cde->n_children = cur->n_children;
 		if (cur->n_children > 0)
 			if (dump_cg_dirs(&cur->children, cur->n_children, &cde->children, strlen(cur->path)) < 0) {