virtio-net: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1402059 ("Missing break in switch")
Addresses-Coverity-ID: 1402060 ("Missing break in switch")
Addresses-Coverity-ID: 1402061 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -697,6 +697,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 			goto xdp_xmit;
 		default:
 			bpf_warn_invalid_xdp_action(act);
+			/* fall through */
 		case XDP_ABORTED:
 			trace_xdp_exception(vi->dev, xdp_prog, act);
 		case XDP_DROP:
@@ -876,8 +877,10 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			goto xdp_xmit;
 		default:
 			bpf_warn_invalid_xdp_action(act);
+			/* fall through */
 		case XDP_ABORTED:
 			trace_xdp_exception(vi->dev, xdp_prog, act);
+			/* fall through */
 		case XDP_DROP:
 			if (unlikely(xdp_page != page))
 				__free_pages(xdp_page, 0);