LU-3050 quota: amend sync write checking from 1.8 client

LU-3050 quota: amend sync write checking from 1.8 client

The 1.8 client sync write doesn't carry OBD_BRW_SYNC flag, to
interoperate with 1.8 client, the checking for sync write on OST
should be amended accordingly.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I4179c4878d295dae625f5631b6b02f3b4dd32cb6
Reviewed-on: http://review.whamcloud.com/5928
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>