selftests/net: Add coverage for UDP GSO with IPv6 extension headers

After enabling UDP GSO for devices not offering checksum offload, we have
hit a regression where a bad offload warning can be triggered when sending
a datagram with IPv6 extension headers.

Extend the UDP GSO IPv6 tests to cover this scenario.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://patch.msgid.link/20240808-udp-gso-egress-from-tunnel-v4-3-f5c5b4149ab9@cloudflare.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>