vlan: reset headers on accel emulation path

It's after all necessary to do reset headers here. The reason is we
cannot depend that it gets reseted in __netif_receive_skb once skb is
reinjected. For incoming vlanids without vlan_dev, vlan_do_receive()
returns false with skb != NULL and __netif_reveive_skb continues, skb is
not reinjected.

This might be good material for 3.0-stable as well

Reported-by: Mike Auty <mike.auty@gmail.com>
Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>