[IPV6]: NDISC: ensure responding to NS without link-layer information

When sending NA in response to NS, we may not know the 
link-layer address for the destination of the NA 
since unicast NS is not required to include its link-layer information.
In this case, we first need to resolve the link-layer address.
(RFC 2461 7.2.4)

We now create neighbour entry for the destination 
and link-layer information will be automatically solved 
in the output path.
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -810,7 +810,8 @@ static void ndisc_recv_ns(struct sk_buff *skb)
 	 *	update / create cache entry
 	 *	for the source address
 	 */
-	neigh = __neigh_lookup(&nd_tbl, saddr, dev, lladdr || !dev->addr_len);
+	neigh = __neigh_lookup(&nd_tbl, saddr, dev,
+			       !inc || lladdr || !dev->addr_len);
 	if (neigh)
 		neigh_update(neigh, lladdr, NUD_STALE, 
 			     NEIGH_UPDATE_F_WEAK_OVERRIDE|