HADOOP-15486. Make NetworkTopology#netLock fair. Contributed by Nanda kumar.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/NetworkTopology.java

@@ -389,7 +389,7 @@ public class NetworkTopology {
   private boolean clusterEverBeenMultiRack = false;
 
   /** the lock used to manage access */
-  protected ReadWriteLock netlock = new ReentrantReadWriteLock();
+  protected ReadWriteLock netlock = new ReentrantReadWriteLock(true);
 
   public NetworkTopology() {
     clusterMap = new InnerNode(InnerNode.ROOT);