vlan: Don't store VLAN tag in cb

Use a real skb member to store the skb to avoid clashes with qdiscs,
which are allowed to use the cb area themselves. As currently only real
devices that consume the skb set the NETIF_F_HW_VLAN_TX flag, no explicit
invalidation is neccessary.

The new member fills a hole on 64 bit, the skb layout changes from:

        __u32                      mark;                 /*   172     4 */
        sk_buff_data_t             transport_header;     /*   176     4 */
        sk_buff_data_t             network_header;       /*   180     4 */
        sk_buff_data_t             mac_header;           /*   184     4 */
        sk_buff_data_t             tail;                 /*   188     4 */
        /* --- cacheline 3 boundary (192 bytes) --- */
        sk_buff_data_t             end;                  /*   192     4 */

        /* XXX 4 bytes hole, try to pack */

to

        __u32                      mark;                 /*   172     4 */
        __u16                      vlan_tci;             /*   176     2 */

        /* XXX 2 bytes hole, try to pack */

        sk_buff_data_t             transport_header;     /*   180     4 */
        sk_buff_data_t             network_header;       /*   184     4 */

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>