netfilter: nf_tables: fix nft_trans type confusion

nft_trans_FOO objects all share a common nft_trans base structure, but
trailing fields depend on the real object size. Access is only safe after
trans->msg_type check.

Check for rule type first.  Found by code inspection.

Fixes: 1a94e38d ("netfilter: nf_tables: add NFTA_RULE_ID attribute")
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nf_tables_api.c

@@ -3865,12 +3865,10 @@ static struct nft_rule *nft_rule_lookup_byid(const struct net *net,
 	struct nft_trans *trans;
 
 	list_for_each_entry(trans, &nft_net->commit_list, list) {
-		struct nft_rule *rule = nft_trans_rule(trans);
-
 		if (trans->msg_type == NFT_MSG_NEWRULE &&
 		    trans->ctx.chain == chain &&
 		    id == nft_trans_rule_id(trans))
-			return rule;
+			return nft_trans_rule(trans);
 	}
 	return ERR_PTR(-ENOENT);
 }