macvtap: do not assume 802.1Q when send vlan packets

The hard-coded 8021.q proto will break 802.1ad traffic. So switch to use
vlan->proto.

Cc: Basil Gor <basil.gor@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>