LU-4388 llite: issue OST_SYNC for fsync()

LU-4388 llite: issue OST_SYNC for fsync()

The last parameter @datasync of fsync() has following indication:
* if datasync=0, we'd always flush data and metadata
* if datasync=1, we'd always flush data while does not flush modifed
  metadata unless that metadata is needed in order to allow a
  subsequent data retrieval to be correctly handled. For example, a
  change to the file size would require a metadata flush.

Lustre client can not tell the difference easily, and would issue
MDS_SYNC and OST_SYNC in all cases.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I90fa8e761e91c8a04afba3fbfafe1f60f5839ad1
Reviewed-on: http://review.whamcloud.com/8684
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>