Initial support for explicit write barriers

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.h b/fio.h
index c2a0d4d..4ed8cb1 100644
--- a/fio.h
+++ b/fio.h
@@ -217,6 +217,7 @@
 	unsigned int thinktime_blocks;
 	unsigned int fsync_blocks;
 	unsigned int fdatasync_blocks;
+	unsigned int barrier_blocks;
 	unsigned long start_delay;
 	unsigned long long timeout;
 	unsigned long long ramp_time;