netfilter: nf_flow_table: add support for sending flows back to the slow path

Since conntrack hasn't seen any packets from the offloaded flow in a
while, and the timeout for offloaded flows is set to an extremely long
value, we need to fix up the state before we can send a flow back to the
slow path.

For TCP, reset td_maxwin in both directions, which makes it resync its
state on the next packets.

Use the regular timeout for TCP and UDP established connections.

This allows the slow path to take over again once the offload state has
been torn down

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