barrier : barrier O_SYNC write incorrect flag check

barrier_writev function was doing the following check to determine whether its a
O_SYNC write or not:
        if (!(flags & O_SYNC))
The problem here is this flag is not fd's flag and gfapi write does not copy
open call fd's flag into write flag because of which O_SYNC writes were not
getting barriered even if barrier was enabled.

The check has been modified as:
        if (!(fd->flags & (O_SYNC | O_DSYNC)))

Change-Id: I07b23852d150b81c7317100ca6d22d082ad897cd
BUG: 1090488
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/7549
Reviewed-by: Varun Shastry <vshastry@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>