ipv6: mark expected switch fall-throughs

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

Notice that in some cases I placed the "fall through" comment
on its own line, which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 9551613..7835dea 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -89,6 +89,7 @@ static bool ip6_tlvopt_unknown(struct sk_buff *skb, int optoff)
 		 */
 		if (ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr))
 			break;
+		/* fall through */
 	case 2: /* send ICMP PARM PROB regardless and drop packet */
 		icmpv6_param_prob(skb, ICMPV6_UNK_OPTION, optoff);
 		return false;