Add support for fdatasync()

Adds a new option, fdatasync=. It's identical to the fsync= option,
but uses fdatasync() instead.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 7c059d1..71ae71d 100644
--- a/fio.h
+++ b/fio.h
@@ -192,6 +192,7 @@
 	unsigned int thinktime_spin;
 	unsigned int thinktime_blocks;
 	unsigned int fsync_blocks;
+	unsigned int fdatasync_blocks;
 	unsigned int start_delay;
 	unsigned long long timeout;
 	unsigned long long ramp_time;