netfilter: nf_tables: unbind callbacks from flowtable destroy path

Callback unbinding needs to be done after nf_flow_table_free(),
otherwise entries are not removed from the hardware.

Update nft_unregister_flowtable_net_hooks() to call
nf_unregister_net_hook() instead since the commit/abort paths do not
deal with the callback unbinding anymore.

Add a comment to nft_flowtable_event() to clarify that
flow_offload_netdev_event() already removes the entries before the
callback unbinding.

Fixes: 8bb69f3b ("netfilter: nf_tables: add flowtable offload control plane")
Fixes ff4bf2f4 ("netfilter: nf_tables: add nft_unregister_flowtable_hook()")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: wenxu <wenxu@ucloud.cn>