Fix disk checker issues reported by kihwal in
There are non-hdfs users of DiskChecker, who use it proactively, not just on failures. This was fine before, but now it incurs heavy I/O due to introduction of fsync() in the code.
HDFS-13538 HDFS DiskChecker should handle disk full situation
- is broken by
HADOOP-13738 DiskChecker should perform some disk IO