HDFS-4994. Audit log getContentSummary() calls. Contributed by Robert Parker.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.23@1516243 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-0.23' into branch-0.23-gd