netfilter: nf_flow_table: tear down TCP flows if RST or FIN was seen

Allow the slow path to handle the shutdown of the connection with proper
timeouts. The packet containing RST/FIN is also sent to the slow path
and the TCP conntrack module will update its state.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>