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: c29f74e0df7a ("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>