fix javadoc.

Author: hfutatzhanghb

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

@@ -136,6 +136,7 @@ public static File idToBlockDir(File root, long blockId) {
    * We hava a block with blockid mapping to:
    * "/data1/hadoop/hdfs/datanode/current/BP-xxxx/current/finalized/subdir0/subdir0"
    * We return "subdir0/subdir0".
+   * @param blockId the block id.
    * @return two-level subdir string where block will be stored.
    */
   public static String idToBlockDirSuffixName(long blockId) {