drbd: correctly submit flush bio on barrier

commit 3899d94e3831ee07ea6821c032dc297aec80586a upstream.

When we receive a flush command (or "barrier" in DRBD), we currently use
a REQ_OP_FLUSH with the REQ_PREFLUSH flag set.

The correct way to submit a flush bio is by using a REQ_OP_WRITE without
any data, and set the REQ_PREFLUSH flag.

Since commit b4a6bb3a67aa ("block: add a sanity check for non-write
flush/fua bios"), this triggers a warning in the block layer, but this
has been broken for quite some time before that.

So use the correct set of flags to actually make the flush happen.

Cc: Christoph Hellwig <[email protected]>
Cc: [email protected]
Fixes: f9ff0da ("drbd: allow parallel flushes for multi-volume resources")
Reported-by: Thomas Voegtle <[email protected]>
Signed-off-by: Christoph Böhmwalder <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/block/drbd/drbd_receiver.c

@@ -1298,7 +1298,7 @@ static void submit_one_flush(struct drbd_device *device, struct issue_flush_cont
 	bio_set_dev(bio, device->ldev->backing_bdev);
 	bio->bi_private = octx;
 	bio->bi_end_io = one_flush_endio;
-	bio->bi_opf = REQ_OP_FLUSH | REQ_PREFLUSH;
+	bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
 
 	device->flush_jif = jiffies;
 	set_bit(FLUSH_PENDING, &device->flags);