HDFS-16871 DiskBalancer process may throw IllegalArgumentException when the target DataNode has capital letter in hostname (#5240)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/diskbalancer/datamodel/DiskBalancerCluster.java

@@ -389,6 +389,6 @@ public class DiskBalancerCluster {
    * @return DiskBalancerDataNode.
    */
   public DiskBalancerDataNode getNodeByName(String hostName) {
-    return hostNames.get(hostName);
+    return hostNames.get(hostName.toLowerCase(Locale.US));
   }
 }