gso: validate gso_type on ipip style tunnels

Commit 121d57af

 ("gso: validate gso_type in GSO handlers") added
gso_type validation to existing gso_segment callback functions, to
filter out illegal and potentially dangerous SKB_GSO_DODGY packets.

Convert tunnels that now call inet_gso_segment and ipv6_gso_segment
directly to have their own callbacks and extend validation to these.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>