test_bpf: fix the dummy skb after dissector changes

Commit d5709f7a

 ("flow_dissector: For stripped vlan, get vlan
info from skb->vlan_tci") made flow dissector look at vlan_proto
when vlan is present.  Since test_bpf sets skb->vlan_tci to ~0
(including VLAN_TAG_PRESENT) we have to populate skb->vlan_proto.

Fixes false negative on test #24:
test_bpf: #24 LD_PAYLOAD_OFF jited:0 175 ret 0 != 42 FAIL (1 times)

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>