ipvs: optimize tunnel dumps for icmp errors

After strip GRE/UDP tunnel header for icmp errors, it's better to show
"GRE/UDP" instead of "IPIP" in debug message.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>