Fix bug that flush doesn't respond to fsync result

Fix bug that flush doesn't respond to fsync result

Summary:
With a regression bug was introduced two years ago, by https://github.com/facebook/rocksdb/commit/6e9fbeb27c38329f33ae541302c44c8db8374f8c , we fail to check return status of fsync call. This can cause we miss the information from the file system and can potentially cause corrupted data which we could have been detected.
Closes https://github.com/facebook/rocksdb/pull/2495

Reviewed By: ajkr

Differential Revision: D5321949

Pulled By: siying

fbshipit-source-id: c68117914bb40700198fc37d0e4c63163a8a1031