virtio_net: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a goto statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/cb9b9534572bc476f4fb7b49a73dc8646b780c84.1605896060.git.gustavoars@kernel.orgSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -732,6 +732,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 			fallthrough;
 		case XDP_ABORTED:
 			trace_xdp_exception(vi->dev, xdp_prog, act);
+			goto err_xdp;
 		case XDP_DROP:
 			goto err_xdp;
 		}