[IPV6]: Send ICMPv6 error on scope violations.

When an IPv6 router is forwarding a packet with a link-local scope source
address off-link, RFC 4007 requires it to send an ICMPv6 destination
unreachable with code 2 ("not neighbor"), but Linux doesn't. Fix below.
Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -463,10 +463,17 @@ int ip6_forward(struct sk_buff *skb)
 		 */
 		if (xrlim_allow(dst, 1*HZ))
 			ndisc_send_redirect(skb, n, target);
-	} else if (ipv6_addr_type(&hdr->saddr)&(IPV6_ADDR_MULTICAST|IPV6_ADDR_LOOPBACK
-						|IPV6_ADDR_LINKLOCAL)) {
+	} else {
+		int addrtype = ipv6_addr_type(&hdr->saddr);
+
 		/* This check is security critical. */
-		goto error;
+		if (addrtype & (IPV6_ADDR_MULTICAST|IPV6_ADDR_LOOPBACK))
+			goto error;
+		if (addrtype & IPV6_ADDR_LINKLOCAL) {
+			icmpv6_send(skb, ICMPV6_DEST_UNREACH,
+				ICMPV6_NOT_NEIGHBOUR, 0, skb->dev);
+			goto error;
+		}
 	}
 
 	if (skb->len > dst_mtu(dst)) {