netfilter: nf_tables: add nft_unregister_flowtable_hook()

Unbind flowtable callback if hook is unregistered.

This patch is implicitly fixing the error path of
nf_tables_newflowtable() and nft_flowtable_event().

Fixes: 8bb69f3b

 ("netfilter: nf_tables: add flowtable offload control plane")
Reported-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>