netfilter: flowtable: Fix flushing of offloaded flows on free

Freeing a flowtable with offloaded flows, the flow are deleted from
hardware but are not deleted from the flow table, leaking them,
and leaving their offload bit on.

Add a second pass of the disabled gc to delete the these flows from
the flow table before freeing it.

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>