[IPV6] NDISC: Avoid updating neighbor cache for proxied address in receiving NA.

This aims at proxying router not updating neighbor cache entry for proxied
address when it receives NA because either the proxied node is off link or
it has already sent a NA to the proxied router.

Based on MIPL2 kernel patch.
Signed-off-by: NVille Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: NMasahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/ndisc.c

@@ -952,6 +952,15 @@ static void ndisc_recv_na(struct sk_buff *skb)
 		if (neigh->nud_state & NUD_FAILED)
 			goto out;
 
+		/*
+		 * Don't update the neighbor cache entry on a proxy NA from
+		 * ourselves because either the proxied node is off link or it
+		 * has already sent a NA to us.
+		 */
+		if (lladdr && !memcmp(lladdr, dev->dev_addr, dev->addr_len) &&
+		    pneigh_lookup(&nd_tbl, &msg->target, dev, 0))
+			goto out;
+
 		neigh_update(neigh, lladdr,
 			     msg->icmph.icmp6_solicited ? NUD_REACHABLE : NUD_STALE,
 			     NEIGH_UPDATE_F_WEAK_OVERRIDE|