selftests/bpf: fix vlan handling in flow dissector program

When we tail call PROG(VLAN) from parse_eth_proto we don't need to peek
back to handle vlan proto because we didn't adjust nhoff/thoff yet. Use
flow_keys->n_proto, that we set in parse_eth_proto instead and
properly increment nhoff as well.

Also, always use skb->protocol and don't look at skb->vlan_present.
skb->vlan_present indicates that vlan information is stored out-of-band
in skb->vlan_{tci,proto} and vlan header is already pulled from skb.
That means, skb->vlan_present == true is not relevant for BPF flow
dissector.

Add simple test cases with VLAN tagged frames:
  * single vlan for ipv4
  * double vlan for ipv6

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>