Added synchronized when iterating over synchronizedList as per Java specification

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NNStorage.java

@@ -836,10 +836,12 @@ public class NNStorage extends Storage implements Closeable,
    * @param sds A list of storage directories to mark as errored.
    */
   void reportErrorsOnDirectories(List<StorageDirectory> sds) {
+    synchronized(sds) {
       for (StorageDirectory sd : sds) {
         reportErrorsOnDirectory(sd);
       }
     }
+  }
 
   /**
    * Reports that a directory has experienced an error.