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

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

BugLink: https://bugs.launchpad.net/bugs/1969110
[ Upstream commit 0492d857636e1c52cd71594a723c4b26a7b31978 ]

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

Fixes: 72efd585f714 ("netfilter: flowtable: add pppoe support")
Fixes: 4cd91f7c290f ("netfilter: flowtable: add vlan support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 2a85c4cb3b1585adad7917dfa7d1889939bdec3e)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>