svn merge -c 1590640 merging from trunk to branch-2 to fix:HDFS-6218. Audit log should use true client IP for proxied webhdfs operations. Contributed by Daryn Sharp.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1590641 13f79535-47bb-0310-9956-ffa450edef68