svn merge -c 1594055 merging from trunk to branch-2 to fix:HDFS-5522. Datanode disk error check may be incorrectly skipped. Contributed by Rushabh Shah.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1594056 13f79535-47bb-0310-9956-ffa450edef68