netfilter: nft_flow_offload: skip tcp rst and fin packets

[ Upstream commit dfe42be15fde16232340b8b2a57c359f51cc10d9 ]

TCP rst and fin packets do not qualify to place a flow into the
flowtable. Most likely there will be no more packets after connection
closure. Without this patch, this flow entry expires and connection
tracking picks up the entry in ESTABLISHED state using the fixup
timeout, which makes this look inconsistent to the user for a connection
that is actually already closed.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>