svn merge -c 1499321 from branch-2 for HDFS-4954. In nfs, OpenFileCtx.getFlushedOffset() should handle IOException.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1499322 13f79535-47bb-0310-9956-ffa450edef68