macvlan: Don't segment multiple tagged packets on macvlan device

Macvlan/macvtap devices don't need to segment multiple tagged packets
since the lower devices can segment them.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>