svn merge -c 1511823 from trunk to branch-2 to FIX HADOOP-9850. RPC kerberos errors don't trigger relogin.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1511824 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd