netfilter: nf_tables: hold mutex on netns pre_exit path

clean_net() runs in workqueue while walking over the lists, grab mutex.

Fixes: 767d1216 ("netfilter: nftables: fix possible UAF over chains from packet path in netns")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -9896,7 +9896,11 @@ static int __net_init nf_tables_init_net(struct net *net)
 
 static void __net_exit nf_tables_pre_exit_net(struct net *net)
 {
+	struct nftables_pernet *nft_net = nft_pernet(net);
+
+	mutex_lock(&nft_net->commit_mutex);
 	__nft_release_hooks(net);
+	mutex_unlock(&nft_net->commit_mutex);
 }
 
 static void __net_exit nf_tables_exit_net(struct net *net)