vlan: Add GRO interfaces

This patch adds GRO interfaces for hardware-assisted VLAN reception.
With this in place we're now at parity with LRO as far as the
interface is concerned.  That is, you can now take any LRO driver
and convert it over to GRO.

As the CB memory clashes with GRO's use of CB, I've removed it
entirely by storing dev in skb->dev.  This is OK because VLAN
gets called first thing in netif_receive_skb and skb->dev is
not used in between us calling netif_rx and netif_receive_skb
getting called.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>