veth: Fix vlan_features so as to be able to use stacked vlan interfaces

Even if we create a stacked vlan interface such as veth0.10.20, it sends
single tagged frames (tagged with only vid 10).
Because vlan_features of a veth interface has the
NETIF_F_HW_VLAN_[CTAG/STAG]_TX bits, veth0.10 also has that feature, so
dev_hard_start_xmit(veth0.10) doesn't call __vlan_put_tag() and
vlan_dev_hard_start_xmit(veth0.10) overwrites vlan_tci.
This prevents us from using a combination of 802.1ad and 802.1Q
in containers, etc.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>