netfilter: nf_tables: disallow binding to already bound chain

Update nft_data_init() to report EINVAL if chain is already bound.

Fixes: d0e2c7de ("netfilter: nf_tables: add NFT_CHAIN_BINDING")
Reported-by: Gwangun Jung <exsociety@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>