selftests/bpf: Add AF_INET packet generation to xdp_metadata

The easiest way to simulate stripped VLAN tag in veth is to send a packet
from VLAN interface, attached to veth. Unfortunately, this approach is
incompatible with AF_XDP on TX side, because VLAN interfaces do not have
such feature.

Check both packets sent via AF_XDP TX and regular socket.

AF_INET packet will also have a filled-in hash type (XDP_RSS_TYPE_L4),
unlike AF_XDP packet, so more values can be checked.

Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231205210847.28460-18-larysa.zaremba@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>