ipvs: fix ipv6 route unreach panic

Previously there was a trivial panic

unshare -n /bin/bash <<EOF
ip addr add dev lo face::1/128
ipvsadm -A -t [face::1]:15213
ipvsadm -a -t [face::1]:15213 -r b00c::1
echo boom | nc face::1 15213
EOF

This patch allows us to replicate the net logic above and simply capture
the skb_dst(skb)->dev and use that for the purpose of the invocation.
Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

net/netfilter/ipvs/ip_vs_xmit.c

@@ -505,6 +505,13 @@ __ip_vs_get_out_rt_v6(int skb_af, struct sk_buff *skb, struct ip_vs_dest *dest,
 	return -1;
 
 err_unreach:
+	/* The ip6_link_failure function requires the dev field to be set
+	 * in order to get the net (further for the sake of fwmark
+	 * reflection).
+	 */
+	if (!skb->dev)
+		skb->dev = skb_dst(skb)->dev;
+
 	dst_link_failure(skb);
 	return -1;
 }