netfilter: nf_tables: GC transaction race with netns dismantle

jira VULN-597
subsystem-sync netfilter:nf_tables 4.18.0-534
commit-author Pablo Neira Ayuso <pablo@netfilter.org>
commit 02c6c24

Use maybe_get_net() since GC workqueue might race with netns exit path.

Fixes: 5f68718 ("netfilter: nf_tables: GC transaction API to avoid race with control plane")
	Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
	Signed-off-by: Florian Westphal <fw@strlen.de>
(cherry picked from commit 02c6c24)
	Signed-off-by: Greg Rose <g.v.rose@ciq.com>

Author: gvrose8192

net/netfilter/nf_tables_api.c

@@ -7902,9 +7902,14 @@ struct nft_trans_gc *nft_trans_gc_alloc(struct nft_set *set,
 	if (!trans)
 		return NULL;
 
+	trans->net = maybe_get_net(net);
+	if (!trans->net) {
+		kfree(trans);
+		return NULL;
+	}
+
 	refcount_inc(&set->refs);
 	trans->set = set;
-	trans->net = get_net(net);
 	trans->seq = gc_seq;
 
 	return trans;
@@ -8161,7 +8166,6 @@ static int nf_tables_commit(struct net *net, struct sk_buff *skb)
 	struct nft_trans_elem *te;
 	struct nft_chain *chain;
 	struct nft_table *table;
-	unsigned int base_seq;
 	LIST_HEAD(adl);
 	int err;