net offloading: Do not mask out NETIF_F_HW_VLAN_TX for vlan.

In netif_skb_features() we return only the features that are valid for vlans
if we have a vlan packet.  However, we should not mask out NETIF_F_HW_VLAN_TX
since it enables transmission of vlan tags and is obviously valid.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>