net: l2tp: mark expected switch fall-through

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

Notice that in this particular case I replaced the "NOBREAK" comment with
a "fall through" comment, which is what GCC is expecting to find.
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_netlink.c

@@ -406,7 +406,7 @@ static int l2tp_nl_tunnel_send(struct sk_buff *skb, u32 portid, u32 seq, int fla
 		if (nla_put_u16(skb, L2TP_ATTR_UDP_SPORT, ntohs(inet->inet_sport)) ||
 		    nla_put_u16(skb, L2TP_ATTR_UDP_DPORT, ntohs(inet->inet_dport)))
 			goto nla_put_failure;
-		/* NOBREAK */
+		/* fall through  */
 	case L2TP_ENCAPTYPE_IP:
 #if IS_ENABLED(CONFIG_IPV6)
 		if (np) {