HDFS-6995. Block should be placed in the client's 'rack-local' node if 'client-local' node is not available (vinayakumarb)
Adding missed test file

(cherry picked from commit cba1f9e3896c0526fa748cd1bb13470d5fae584a)
HDFS-7242. Code improvement for FSN#checkUnreadableBySuperuser. (Contributed by Yi Liu)

(cherry picked from commit 1c3ff0b7c892b9d70737c375fb6f4a6fc6dd6d81)
HDFS-7252. small refinement to the use of isInAnEZ in FSNamesystem. (Yi Liu via vinayakumarb)

(cherry picked from commit 368743140dd076ecd5af309c1ed83c5ae2d59fc8)