cgroup2: Manager.Delete: handle both "threaded" and "domain threaded"

commit 6f5001d added special handling
for threaded cgroup types. A later contribution added detection for
"domain threaded" as known type, but did not update the handling to
detect this type.

From the original PR;

> Reading cgroup.procs seems to return ENOTSUPP when threaded, so check
> the type of the cg when going to delete and read the relevant file.

An alternative could be to check both variants unconditionally, and to
error if either Manager.Threads or Manager.Procs is non-zero.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cgroup2/manager.go

@@ -472,10 +472,6 @@ func (c *Manager) fallbackKill() error {
 }
 
 func (c *Manager) Delete() error {
-	var (
-		tasks    []uint64
-		threaded bool
-	)
 	// Kernel prevents cgroups with running process from being removed,
 	// check the tree is empty.
 	//
@@ -485,13 +481,13 @@ func (c *Manager) Delete() error {
 		if !os.IsNotExist(err) {
 			return err
 		}
-	} else {
-		threaded = cgType == Threaded
 	}
 
-	if threaded {
+	var tasks []uint64
+	switch cgType {
+	case Threaded, DomainThreaded:
 		tasks, err = c.Threads(true)
-	} else {
+	default:
 		tasks, err = c.Procs(true)
 	}
 	if err != nil {