move the change log of HDFS-637 from trunk to the 0.21 section

git-svn-id: https://svn.apache.org/repos/asf/hadoop/hdfs/trunk@818298 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -11,9 +11,7 @@ Trunk (unreleased changes)
   OPTIMIZATIONS
 
   BUG FIXES
-  HDFS-640. Fixed TestHDFSFileContextMainOperations.java build failure. (suresh)
-
-  HDFS-637. DataNode sends a Success ack when block write fails. (hairong)
+    HDFS-640. Fixed TestHDFSFileContextMainOperations.java build failure. (suresh)
 
 Release 0.21.0 - Unreleased
 
@@ -284,6 +282,8 @@ Release 0.21.0 - Unreleased
     HDFS-629. Remove ReplicationTargetChooser.java along with fixing 
     import warnings generated by Eclipse. (dhruba)
 
+    HDFS-637. DataNode sends a Success ack when block write fails. (hairong)
+
 Release 0.20.1 - 2009-09-01
 
   IMPROVEMENTS