vlan: allow TSO setting on vlan interfaces

When we need to shape traffic using low speeds, we need to
disable tso on network interface :

ethtool -K eth0.2240 tso off

It seems vlan interfaces miss the set_tso() ethtool method.

Before enabling TSO, we must check real device supports
TSO for VLAN-tagged packets and enables TSO.

Note that a TSO change on real device propagates TSO setting
on all vlans, even if admin selected a different TSO setting.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>