dgram: refactor to remove unreachable code

There is no need to ensure `cluster` loaded in `bindServerHandle` because
it's already loaded before calling it.

This PR makes `cluster` as an argument to remind developers that `cluster`
needs to be loaded before calling this function.

lib/dgram.js

@@ -199,10 +199,7 @@ function bufferSize(self, size, buffer) {
 }
 
 // Query primary process to get the server handle and utilize it.
-function bindServerHandle(self, options, errCb) {
-  if (!cluster)
-    cluster = require('cluster');
-
+function bindServerHandle(cluster, self, options, errCb) {
   const state = self[kStateSymbol];
   cluster._getServer(self, options, (err, handle) => {
     if (err) {
@@ -266,7 +263,7 @@ Socket.prototype.bind = function(port_, address_ /* , callback */) {
       cluster = require('cluster');
 
     if (cluster.isWorker && !exclusive) {
-      bindServerHandle(this, {
+      bindServerHandle(cluster, this, {
         address: null,
         port: null,
         addressType: this.type,
@@ -335,7 +332,7 @@ Socket.prototype.bind = function(port_, address_ /* , callback */) {
       flags |= UV_UDP_IPV6ONLY;
 
     if (cluster.isWorker && !exclusive) {
-      bindServerHandle(this, {
+      bindServerHandle(cluster, this, {
         address: ip,
         port: port,
         addressType: this.type,