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

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

@@ -105,7 +105,7 @@ public class NetworkTopology {
   private boolean clusterEverBeenMultiRack = false;
 
   /** the lock used to manage access */
-  protected ReadWriteLock netlock = new ReentrantReadWriteLock();
+  protected ReadWriteLock netlock = new ReentrantReadWriteLock(true);
 
   // keeping the constructor because other components like MR still uses this.
   public NetworkTopology() {