Add cgroup delete opt for ns deletion

Closes #3305

This adds an opt and a `--cgroup,-c` flag to `ctr namespaces rm` to
remove the cgroup that is commonly created with runtimes.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

namespaces_opts_linux.go

@@ -0,0 +1,36 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package containerd
+
+import (
+	"context"
+
+	"github.com/containerd/cgroups"
+	"github.com/containerd/containerd/namespaces"
+)
+
+// WithNamespaceCgroupDeletion removes the cgroup directory that was created for the namespace
+func WithNamespaceCgroupDeletion(ctx context.Context, i *namespaces.DeleteInfo) error {
+	cg, err := cgroups.Load(cgroups.V1, cgroups.StaticPath(i.Name))
+	if err != nil {
+		if err == cgroups.ErrCgroupDeleted {
+			return nil
+		}
+		return err
+	}
+	return cg.Delete()
+}