vlan: Update skb->mac_header in __vlan_put_tag().

After moving mac addresses in __vlan_put_tag() skb->mac_header needs
to be updated.

Reported-by: Karl Hiramoto <karl@hiramoto.org>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>