net: fix xfrm reverse flow lookup for icmp6

This patch fixes the xfrm reverse flow lookup for icmp6 so that icmp6 packets
don't get lost over ipsec tunnels. Similar patch is in RHEL5 kernel for a quite
long time and I do not see why it isn't in mainline.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>