LU-4366 test: sync didn't return ENOMEM

LU-4366 test: sync didn't return ENOMEM

The fail_loc has been intercepted by earlier write, this patch add
a sync before setting the fail_loc.

Test-Parameters: alwaysuploadlogs testlist=sanity
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I0517ccc591651e275dc4d4cd2a3de179c2c2b3f1
Reviewed-on: http://review.whamcloud.com/14658
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>