HDFS-7430 backport addendum. Add missing change. (cmccabe)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java

@@ -382,6 +382,7 @@ public class DataNode extends ReconfigurableBase
            final List<StorageLocation> dataDirs,
            final SecureResources resources) throws IOException {
     super(conf);
+    this.blockScanner = new BlockScanner(this, conf);
     this.lastDiskErrorCheck = 0;
     this.maxNumberOfBlocksToLog = conf.getLong(DFS_MAX_NUM_BLOCKS_TO_LOG_KEY,
         DFS_MAX_NUM_BLOCKS_TO_LOG_DEFAULT);