netfilter: nf_tables_offload: fix check the chain offload flag

In the nft_indr_block_cb the chain should check the flag with
NFT_CHAIN_HW_OFFLOAD.

Fixes: 9a32669f ("netfilter: nf_tables_offload: support indr block call")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>