netfilter: nf_tables: free chain context when BINDING flag is missing

syzbot found a memory leak in nf_tables_addchain() because the chain
object is not free'd correctly on error.

Fixes: d0e2c7de ("netfilter: nf_tables: add NFT_CHAIN_BINDING")
Reported-by:  <syzbot+c99868fde67014f7e9f5@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>