src/fsync-err: write to different offset on each fd

src/fsync-err: write to different offset on each fd

NFS currently has slightly different semantics from other fs' and
fails this test due to the fact that the same range is overwritten
via each fd.

Change it so that each fd overwrites a different region, which is
more representative of a real workload anyway.

Reported-by: Neil Brown <neilb@suse.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>