drbd: Silenced an assert that could triggered after changing write ordering method

Immediately after changing the write ordering method, the epoch can already
be finished at this point.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -1201,10 +1201,11 @@ static int receive_Barrier(struct drbd_conf *mdev, struct p_header *h)
 
 	case WO_bdev_flush:
 	case WO_drain_io:
-		D_ASSERT(rv == FE_STILL_LIVE);
-		set_bit(DE_BARRIER_IN_NEXT_EPOCH_ISSUED, &mdev->current_epoch->flags);
-		drbd_wait_ee_list_empty(mdev, &mdev->active_ee);
-		rv = drbd_flush_after_epoch(mdev, mdev->current_epoch);
+		if (rv == FE_STILL_LIVE) {
+			set_bit(DE_BARRIER_IN_NEXT_EPOCH_ISSUED, &mdev->current_epoch->flags);
+			drbd_wait_ee_list_empty(mdev, &mdev->active_ee);
+			rv = drbd_flush_after_epoch(mdev, mdev->current_epoch);
+		}
 		if (rv == FE_RECYCLED)
 			return TRUE;