ipvs: strip udp tunnel headers from icmp errors

Recognize UDP tunnels in received ICMP errors and
properly strip the tunnel headers. GUE is what we
have for now.

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>