netfilter: nf_flow_table: add a new flow state for tearing down offloading

On cleanup, this will be treated differently from FLOW_OFFLOAD_DYING:

If FLOW_OFFLOAD_DYING is set, the connection is going away, so both the
offload state and the connection tracking entry will be deleted.

If FLOW_OFFLOAD_TEARDOWN is set, the connection remains alive, but
the offload state is torn down. This is useful for cases that require
more complex state tracking / timeout handling on TCP, or if the
connection has been idle for too long.

Support for sending flows back to the slow path will be implemented in
a following patch

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