HDFS-14497. Addendum: Write lock held by metasave impact following RPC processing.

Signed-off-by: Wei-Chiu Chuang <weichiu@apache.org>
(cherry picked from commit dde9399b37bffb77da17c025f0b9b673d7088bc6)

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

@@ -594,7 +594,7 @@ public class FSNamesystem implements Namesystem, FSNamesystemMBean,
    * HDFS-14497: Concurrency control when many metaSave request to write
    * meta to same out stream after switch to read lock.
    */
-  private Object metaSaveLock = new Object();
+  private final Object metaSaveLock = new Object();
 
   /**
    * Notify that loading of this FSDirectory is complete, and