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: NKarl Hiramoto <karl@hiramoto.org>
Signed-off-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/if_vlan.h

@@ -210,6 +210,7 @@ static inline struct sk_buff *__vlan_put_tag(struct sk_buff *skb, u16 vlan_tci)
 
 	/* Move the mac addresses to the beginning of the new header. */
 	memmove(skb->data, skb->data + VLAN_HLEN, 2 * VLAN_ETH_ALEN);
+	skb->mac_header -= VLAN_HLEN;
 
 	/* first, the ethernet type */
 	veth->h_vlan_proto = htons(ETH_P_8021Q);