geneve: fix needed_headroom and max_mtu for collect_metadata

Since commit 9b4437a5 ("geneve: Unify LWT and netdev handling.")
when using COLLECT_METADATA geneve devices are created with too small of
a needed_headroom and too large of a max_mtu. This is because
ip_tunnel_info_af() is not valid with the device level info when using
COLLECT_METADATA and we mistakenly fall into the IPv4 case.

For COLLECT_METADATA, always use the worst case of ipv6 since both
sockets are created.

Fixes: 9b4437a5 ("geneve: Unify LWT and netdev handling.")
Signed-off-by: Eric Garver <e@erig.me>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>