ipv6: icmp6_send: fix Oops when pinging a not set up IPv6 peer on a sit tunnel

The patch 446fab5933 ("ipv6: enable anycast addresses
as source addresses in ICMPv6 error messages") causes an Oops when pinging a not
set up IPv6 peer on a sit tunnel.

The problem is that ipv6_anycast_destination() uses unconditionally skb_dst(skb),
which is NULL in this case.

The solution is to use instead the ipv6_chk_acast_addr_src() function.

Here are the steps to reproduce it:
modprobe sit
ip link add sit1 type sit remote 10.16.0.121 local 10.16.0.249
ip l s sit1 up
ip -6 a a dev sit1 2001🔢:123 remote 2001🔢:121
ping6 2001🔢:121

Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Francois-Xavier Le Bail <fx.lebail@yahoo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/icmp.c

@@ -414,7 +414,7 @@ static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info)
 	addr_type = ipv6_addr_type(&hdr->daddr);
 
 	if (ipv6_chk_addr(net, &hdr->daddr, skb->dev, 0) ||
-	    ipv6_anycast_destination(skb))
+	    ipv6_chk_acast_addr_src(net, skb->dev, &hdr->daddr))
 		saddr = &hdr->daddr;
 
 	/*