YARN-1082. Addendum patch.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1516352 13f79535-47bb-0310-9956-ffa450edef68

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/recovery/FileSystemRMStateStore.java

@@ -91,7 +91,7 @@ public class FileSystemRMStateStore extends RMStateStore {
   }
 
   @Override
-  protected void startInternal() throws Exception {
+  protected synchronized void startInternal() throws Exception {
     // create filesystem only now, as part of service-start. By this time, RM is
     // authenticated with kerberos so we are good to create a file-system
     // handle.