bpf: selftests: send packet to devmap redirect XDP

The current xdp_devmap_attach test attaches a program
that redirects to another program via devmap.

It is, however, never executed, so do that to catch
any bugs that might occur during execution.

Also, execute the same for a veth pair so that we
also cover the non-generic path.

Warning: Running this without the bugfix in this series
will likely crash your system.

Signed-off-by: Florian Kauer <florian.kauer@linutronix.de>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20240911-devel-koalo-fix-ingress-ifindex-v4-2-5c643ae10258@linutronix.de


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>