netfilter: flowtable: Fix setting forgotten NF_FLOW_HW_DEAD flag

During the refactor this was accidently removed.

Fixes: ae290450

 ("netfilter: flowtable: add nf_flow_offload_tuple() helper")
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>