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/HOWTO b/HOWTO
index 0eab6e1..f323c20 100644
--- a/HOWTO
+++ b/HOWTO
@@ -530,6 +530,9 @@
 		not sync the file. The exception is the sg io engine, which
 		synchronizes the disk cache anyway.
 
+fsyncdata=int	Like fsync= but uses fdatasync() to only sync data and not
+		metadata blocks.
+
 overwrite=bool	If true, writes to a file will always overwrite existing
 		data. If the file doesn't already exist, it will be
 		created before the write phase begins. If the file exists