netfilter: nf_tables_offload: unbind if multi-device binding fails

nft_flow_block_chain() needs to unbind in case of error when performing
the multi-device binding.

Fixes: d54725cd

 ("netfilter: nf_tables: support for multiple devices per netdev hook")
Reported-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>