geneve: fix ip_hdr_len reserved for geneve6 tunnel.

It shold reserved sizeof(ipv6hdr) for geneve in ipv6 tunnel.

Fixes: c3ef5aa5

 ('geneve: Merge ipv4 and ipv6 geneve_build_skb()')
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>