NameNodeConnector should prevent other balancers to run while the one is running

In hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/NameNodeConnector.java:

private OutputStream checkAndMarkRunningBalancer() throws IOException {
try

{ final DataOutputStream out = fs.create(BALANCER_ID_PATH); out.writeBytes(InetAddress.getLocalHost().getHostName()); out.flush(); return out; }

catch(RemoteException e) {
...

In the code, fs.create should set overwrite bit to false so that other balancer can't run while one is running.