datapath: Update hardware computed checksum on VLAN change.

datapath: Update hardware computed checksum on VLAN change.

The checksum computed by hardware on receive stored in skb->csum
when skb->ip_summed == CHECKSUM_COMPLETE is supposed to reflect
the contents of the packet starting at skb->data, which includes
the VLAN tag if there is one.  However, when we manipulate the
VLAN tag we don't update the checksum.  This leads to all kinds
of nasty warnings about broken hardware, not to mention we can't
take advantage of the checksum that was already computed.

This also fixes some issues with our private checksum type value
on some different kernels and after GSO.