netfilter: flowtable: Fix QinQ and pppoe support for inet table

nf_flow_offload_inet_hook() does not check for 802.1q and PPPoE.
Fetch inner ethertype from these encapsulation protocols.

Fixes: 72efd585 ("netfilter: flowtable: add pppoe support")
Fixes: 4cd91f7c

 ("netfilter: flowtable: add vlan support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>