netfilter: flowtable: add pppoe support

Add the PPPoE protocol and session id to the flow tuple using the encap
fields to uniquely identify flows from the receive path. For the
transmit path, dev_hard_header() on the vlan device push the headers.

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