macvtap: introduce macvtap_get_vlan()

Factor out the device holding logic to a macvtap_get_vlan(), this will be also
used by multiqueue API.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>