netfilter: flowtable: incorrect pppoe tuple

[ Upstream commit 6db5dc7b ]

pppoe traffic reaching ingress path does not match the flowtable entry
because the pppoe header is expected to be at the network header offset.
This bug causes a mismatch in the flow table lookup, so pppoe packets
enter the classical forwarding path.

Fixes: 72efd585

 ("netfilter: flowtable: add pppoe support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>