netfilter: nf_tables: free flowtable hooks on hook register error

If hook registration fails, the hooks allocated via nft_netdev_hook_alloc
need to be freed.

We can't change the goto label to 'goto 5' -- while it does fix the memleak
it does cause a warning splat from the netfilter core (the hooks were not
registered).

Fixes: 3f0465a9 ("netfilter: nf_tables: dynamically allocate hooks per net_device in flowtables")
Reported-by: syzbot+a2ff6fa45162a5ed4dd3@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -6300,8 +6300,13 @@ static int nf_tables_newflowtable(struct net *net, struct sock *nlsk,
 		goto err4;
 
 	err = nft_register_flowtable_net_hooks(ctx.net, table, flowtable);
-	if (err < 0)
+	if (err < 0) {
+		list_for_each_entry_safe(hook, next, &flowtable->hook_list, list) {
+			list_del_rcu(&hook->list);
+			kfree_rcu(hook, rcu);
+		}
 		goto err4;
+	}
 
 	err = nft_trans_flowtable_add(&ctx, NFT_MSG_NEWFLOWTABLE, flowtable);
 	if (err < 0)