netfilter: flowtable: Fix missing flush hardware on table free

If entries exist when freeing a hardware offload enabled table,
we queue work for hardware while running the gc iteration.

Execute it (flush) after queueing.

Fixes: c29f74e0

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