net/packet: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break 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>

net/packet/af_packet.c

@@ -1655,6 +1655,7 @@ static int fanout_add(struct sock *sk, struct fanout_args *args)
 	case PACKET_FANOUT_ROLLOVER:
 		if (type_flags & PACKET_FANOUT_FLAG_ROLLOVER)
 			return -EINVAL;
+		break;
 	case PACKET_FANOUT_HASH:
 	case PACKET_FANOUT_LB:
 	case PACKET_FANOUT_CPU: