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/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/cb[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/net/virtio_net.c

···
 			fallthrough;
 		case XDP_ABORTED:
 			trace_xdp_exception(vi->dev, xdp_prog, act);
+			goto err_xdp;
 		case XDP_DROP:
 			goto err_xdp;
 		}