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: f80a612dd77c ("netfilter: nf_tables: add support to destroy operation")
Signed-off-by: Yang Yingliang <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: Fernando Fernandez Mancera <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_tables_api.c

···
 		chain = nft_chain_lookup(net, table, nla[NFTA_RULE_CHAIN],
 					 genmask);
 		if (IS_ERR(chain)) {
-			if (PTR_ERR(rule) == -ENOENT &&
+			if (PTR_ERR(chain) == -ENOENT &&
 			    NFNL_MSG_TYPE(info->nlh->nlmsg_type) == NFT_MSG_DESTROYRULE)
 				return 0;