[vfs2] Don't take reference in Task.MountNamespaceVFS2 and MountNamespace.Root.

This fixes reference leaks related to accidentally forgetting to DecRef()
after calling one or the other.

PiperOrigin-RevId: 336918922

pkg/sentry/fsimpl/devtmpfs/devtmpfs.go

@@ -95,10 +95,13 @@ func NewAccessor(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth
 	if err != nil {
 		return nil, err
 	}
+	// Pass a reference on root to the Accessor.
+	root := mntns.Root()
+	root.IncRef()
 	return &Accessor{
 		vfsObj: vfsObj,
 		mntns:  mntns,
-		root:   mntns.Root(),
+		root:   root,
 		creds:  creds,
 	}, nil
 }

pkg/sentry/fsimpl/devtmpfs/devtmpfs_test.go

@@ -53,6 +53,7 @@ func setupDevtmpfs(t *testing.T) (context.Context, *auth.Credentials, *vfs.Virtu
 		t.Fatalf("failed to create tmpfs root mount: %v", err)
 	}
 	root := mntns.Root()
+	root.IncRef()
 	devpop := vfs.PathOperation{
 		Root:  root,
 		Start: root,