net: icmp6_send should use dst dev to determine L3 domain

icmp6_send is called in response to some event. The skb may not have
the device set (skb->dev is NULL), but it is expected to have a dst set.
Update icmp6_send to use the dst on the skb to determine L3 domain.

Fixes: ca254490 ("net: Add VRF support to IPv6 stack")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/icmp.c

@@ -448,7 +448,7 @@ static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 	if (__ipv6_addr_needs_scope_id(addr_type))
 		iif = skb->dev->ifindex;
 	else
-		iif = l3mdev_master_ifindex(skb->dev);
+		iif = l3mdev_master_ifindex(skb_dst(skb)->dev);
 
 	/*
 	 *	Must not send error if the source does not uniquely