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.1 b/fio.1
index 637304e..fc055e4 100644
--- a/fio.1
+++ b/fio.1
@@ -365,6 +365,10 @@
 How many I/Os to perform before issuing an \fBfsync\fR\|(2) of dirty data.  If
 0, don't sync.  Default: 0.
 .TP
+.BI fdatasync \fR=\fPint
+Like \fBfsync\fR, but uses \fBfdatasync\fR\|(2) instead to only sync the
+data parts of the file. Default: 0.
+.TP
 .BI overwrite \fR=\fPbool
 If writing, setup the file first and do overwrites.  Default: false.
 .TP