HDFS-5889. When starting rolling upgrade, create a fs image for rollback so that the standby namenode can create checkpoints during upgrade.  Contributed by szetszwo & jing9


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-5535@1567861 13f79535-47bb-0310-9956-ffa450edef68