ipvs: add function to find tunnels

Add ip_vs_find_tunnel() to match tunnel headers
by family, address and optional port. Use it to
properly find the tunnel real server used in
received ICMP errors.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>