selftests/net: Add test for loading devbound XDP program in generic mode

Add a test to bpf_offload.py for loading a devbound XDP program in
generic mode, checking that it fails correctly.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250127131344.238147-2-toke@redhat.com


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