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>