Move Flatten INode hierarchy jiras (HDFS-5531, HDFS-5285, HDFS-5286, HDFS-5537, HDFS-5554, HDFS-5647, HDFS-5632, HDFS-5715, HDFS-5726) to 2.4.0 section in CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1568218 13f79535-47bb-0310-9956-ffa450edef68