vfs: MountNamespace.Root() has to return a top mount of /

A few mounts can be mounted on top of `/`.

PiperOrigin-RevId: 558264274

pkg/sentry/fsimpl/devtmpfs/devtmpfs.go

@@ -96,8 +96,7 @@ func NewAccessor(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth
 		return nil, err
 	}
 	// Pass a reference on root to the Accessor.
-	root := mntns.Root()
-	root.IncRef()
+	root := mntns.Root(ctx)
 	return &Accessor{
 		vfsObj: vfsObj,
 		mntns:  mntns,

pkg/sentry/fsimpl/devtmpfs/devtmpfs_test.go

@@ -52,8 +52,7 @@ func setupDevtmpfs(t *testing.T) (context.Context, *auth.Credentials, *vfs.Virtu
 	if err != nil {
 		t.Fatalf("failed to create tmpfs root mount: %v", err)
 	}
-	root := mntns.Root()
-	root.IncRef()
+	root := mntns.Root(ctx)
 	devpop := vfs.PathOperation{
 		Root:  root,
 		Start: root,