HDFS-6705. Create an XAttr that disallows the HDFS admin from accessing a file. (clamb via wang)

(cherry picked from commit ea4e2e843ecadd8019ea35413f4a34b97a424923)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java