utils: move the cgroup if root on cgroupv2

if we are running on cgroupv2, force the creation of a sub-cgroup even
when we are at the root for the cgroup v2 unified mount.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

utils/utils_supported.go

@@ -137,7 +137,7 @@ func moveUnderCgroup(cgroup, subtree string, processes []uint32) error {
 		}
 
 		// root cgroup, skip it
-		if parts[2] == "/" {
+		if parts[2] == "/" && !(unifiedMode && parts[1] == "") {
 			continue
 		}