net/sched: act_ct: remove and free nf_table callbacks

When cleaning up the nf_table in tcf_ct_flow_table_cleanup_work
there is no guarantee that the callback list, added to by
nf_flow_table_offload_add_cb, is empty. This means that it is
possible that the flow_block_cb memory allocated will be lost.

Fix this by iterating the list and freeing the flow_block_cb entries
before freeing the nf_table entry (via freeing ct_ft).

Fixes: 978703f4 ("netfilter: flowtable: Add API for registering to flow table events")
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>