vlan: Add ability to always enable TSO/UFO

Since the real device can segment packets by software, a vlan device
can set TSO/UFO even when the real device doesn't have those features.
Unlike GSO, this allows packets to be segmented after Qdisc.

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