[IPV6]: Make ndisc_dst_alloc() common for later use.

For later use, this patch is renaming ndisc_dst_alloc()
(and related function/structures) to icmp6_dst_alloc()
(and so on).  This patch also removing unused function-
pointer argument for it.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/ndisc.c

@@ -462,7 +462,7 @@ static void __ndisc_send(struct net_device *dev,
 	icmpv6_flow_init(ndisc_socket->sk, &fl, type,
 			 saddr, daddr, dev->ifindex);
 
-	dst = ndisc_dst_alloc(dev, neigh, daddr, ip6_output);
+	dst = icmp6_dst_alloc(dev, neigh, daddr);
 	if (!dst)
 		return;