netfilter: nft_flow_offload: fix offload with pppoe + vlan

[ Upstream commit 24560749 ]

When running a combination of PPPoE on top of a VLAN, we need to set
info->outdev to the PPPoE device, otherwise PPPoE encap is skipped
during software offload.

Fixes: 72efd585

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