storage/posix: fix issue in posix_fsync

Fix the issue that posix_fsync does not correctly return and save
 error code in op_errno when call to sys_fdatasync fails.

Change-Id: Id0b62cfa009dbb52c8a0992abd5c46330fa0a8c0
BUG: 1125814
Signed-off-by: Zhang Huan <zhhuan@gmail.com>
Reviewed-on: http://review.gluster.org/8398
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>