selftests/net: amend bpf flow dissector prog to do vnet hdr validation

Change the BPF flow dissector program to perform various checks on the
virtio_net_hdr fields after doing flow dissection.

Amend test_flow_dissector.(c|sh) to add test cases that inject packets
with reasonable or unreasonable virtio-net headers and assert that bad
packets are dropped and good packets are not. Do this via packet
socket; the kernel executes tpacket_snd, which enters
virtio_net_hdr_to_skb, where flow dissection / vnet header validation
occurs.

Changes
v4:
  - Read virtio_net_hdr pointer from struct bpf_flow_keys
  - Add vnet header endianness logic to BPF program

Signed-off-by: Tanner Love <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>

Author: tannerlove