code: func addNodeSelectorToPodSpec(podSpec *v1.PodSpec) {
  	if podSpec.NodeSelector == nil {
  		podSpec.NodeSelector = map[string]string{kubeadmconstants.LabelNodeRoleMaster: ""}
  		return
  	}
  
  	podSpec.NodeSelector[kubeadmconstants.LabelNodeRoleMaster] = ""
  }
docs: // addNodeSelectorToPodSpec makes Pod require to be scheduled on a node marked with the control-plane label