vlan: fix network_header/mac_header adjustments

Lennert Buytenhek points out that the VLAN code incorrectly adjusts
skb->network_header to point in the middle of the VLAN header and
additionally tries to adjust skb->mac_header without checking for
validity.

The network_header should not be touched at all since we're only
adding headers in front of it, mac_header adjustments are not
necessary at all.

Based on patch by Lennert Buytenhek <buytenh@wantstofly.org>.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_vlan.h

@@ -279,8 +279,6 @@ static inline struct sk_buff *__vlan_put_tag(struct sk_buff *skb, unsigned short
 	veth->h_vlan_TCI = htons(tag);
 
 	skb->protocol = htons(ETH_P_8021Q);
-	skb->mac_header -= VLAN_HLEN;
-	skb->network_header -= VLAN_HLEN;
 
 	return skb;
 }

net/8021q/vlan_dev.c

@@ -308,7 +308,6 @@ static int vlan_dev_hard_header(struct sk_buff *skb, struct net_device *dev,
 			vhdr->h_vlan_encapsulated_proto = htons(len);
 
 		skb->protocol = htons(ETH_P_8021Q);
-		skb_reset_network_header(skb);
 	}
 
 	/* Before delegating work to the lower layer, enter our MAC-address */