netfilter: nf_tables: fix wrong pointer passed to PTR_ERR()

It should be 'chain' passed to PTR_ERR() in the error path
after calling nft_chain_lookup() in nf_tables_delrule().

Fixes: f80a612d ("netfilter: nf_tables: add support to destroy operation")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>