drbd: we should write meta data updates with FLUSH FUA

We used to write these with BIO_RW_BARRIER aka REQ_HARDBARRIER (unless
disabled in the configuration). The correct semantic now would be to
write with FLUSH/FUA.
For example, with activity log transactions, FUA alone is not enough, we
need the corresponding bitmap update (and all related application
updates) on stable storage as well.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_actlog.c

@@ -79,7 +79,7 @@ static int _drbd_md_sync_page_io(struct drbd_conf *mdev,
 	md_io.error = 0;
 
 	if ((rw & WRITE) && !test_bit(MD_NO_FUA, &mdev->flags))
-		rw |= REQ_FUA;
+		rw |= REQ_FUA | REQ_FLUSH;
 	rw |= REQ_SYNC;
 
 	bio = bio_alloc(GFP_NOIO, 1);