netfilter: flowtable: set NF_FLOW_TEARDOWN flag on entry expiration

If the flow timer expires, the gc sets on the NF_FLOW_TEARDOWN flag.
Otherwise, the flowtable software path might race to refresh the
timeout, leaving the state machine in inconsistent state.

Fixes: c29f74e0

 ("netfilter: nf_flow_table: hardware offload support")
Reported-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>