veth: Mark expected switch fall-throughs

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

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -540,8 +540,10 @@ static struct sk_buff *veth_xdp_rcv_one(struct veth_rq *rq,
 			goto xdp_xmit;
 		default:
 			bpf_warn_invalid_xdp_action(act);
+			/* fall through */
 		case XDP_ABORTED:
 			trace_xdp_exception(rq->dev, xdp_prog, act);
+			/* fall through */
 		case XDP_DROP:
 			goto err_xdp;
 		}
@@ -661,8 +663,10 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq, struct sk_buff *skb,
 		goto xdp_xmit;
 	default:
 		bpf_warn_invalid_xdp_action(act);
+		/* fall through */
 	case XDP_ABORTED:
 		trace_xdp_exception(rq->dev, xdp_prog, act);
+		/* fall through */
 	case XDP_DROP:
 		goto drop;
 	}