Moved CHANGES.txt entries of MAPREDUCE-5898, MAPREDUCE-5920, HDFS-6464, HDFS-6375 from trunk to 2.5 section on merging HDFS-2006 to branch-2

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1602699 13f79535-47bb-0310-9956-ffa450edef68