so after it, we should recompute the checksum to include these 4 bytes.
skb->data still points to the mac header, therefore VLAN header is at
(2 * ETH_ALEN = 12) bytes after it, not (ETH_HLEN = 14) bytes.
Therefore the VLAN_HLEN = 4 bytes after 2 * ETH_ALEN is the part
we want to sub from checksum.
Cc: David S. Miller <davem@davemloft.net> Cc: Jesse Gross <jesse@nicira.com> Signed-off-by: Cong Wang <amwang@redhat.com> Signed-off-by: Jesse Gross <jesse@nicira.com>