selftests/bpf: Make sure zero-len skbs aren't redirectable

LWT_XMIT to test L3 case, TC to test L2 case.

v2:
- s/veth_ifindex/ipip_ifindex/ in two places (Martin)
- add comment about which condition triggers the rejection (Martin)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20221121180340.1983627-2-sdf@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>