netfilter: flowtable: add vlan support

Add the vlan id and protocol to the flow tuple to uniquely identify
flows from the receive path. For the transmit path, dev_hard_header() on
the vlan device push the headers. This patch includes support for two
vlan headers (QinQ) from the ingress path.

Add a generic encap field to the flowtable entry which stores the
protocol and the tag id. This allows to reuse these fields in the PPPoE
support coming in a later patch.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>