selftests: net: tests for add double tunneling GRO/GSO

selftests: net: tests for add double tunneling GRO/GSO

Create a simple, netns-based topology with double, nested UDP tunnels and
perform TSO transfers on top.

Explicitly enable GSO and/or GRO and check the skb layout consistency with
different configuration allowing (or not) GSO frames to be delivered on
the other end.

The trickest part is account in a robust way the aggregated/unaggregated
packets with double encapsulation: use a classic bpf filter for it.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/61f2c98ba0f73057c2d6f6cb62eb807abd90bf6b.1769011015.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>