netfilter: nft_set_pipapo: make pipapo_clone helper return NULL

Currently it returns an error pointer, but the only possible failure
is ENOMEM.

After a followup patch, we'd need to discard the errno code, i.e.

x = pipapo_clone()
if (IS_ERR(x))
	return NULL

or make more changes to fix up callers to expect IS_ERR() code
from set->ops->deactivate().

So simplify this and make it return ptr-or-null.
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_set_pipapo.c

@@ -1395,7 +1395,7 @@ static int nft_pipapo_insert(const struct net *net, const struct nft_set *set,
  * pipapo_clone() - Clone matching data to create new working copy
  * @old:	Existing matching data
  *
- * Return: copy of matching data passed as 'old', error pointer on failure
+ * Return: copy of matching data passed as 'old' or NULL.
  */
 static struct nft_pipapo_match *pipapo_clone(struct nft_pipapo_match *old)
 {
@@ -1405,7 +1405,7 @@ static struct nft_pipapo_match *pipapo_clone(struct nft_pipapo_match *old)
 
 	new = kmalloc(struct_size(new, f, old->field_count), GFP_KERNEL);
 	if (!new)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
 	new->field_count = old->field_count;
 	new->bsize_max = old->bsize_max;
@@ -1477,7 +1477,7 @@ static struct nft_pipapo_match *pipapo_clone(struct nft_pipapo_match *old)
 	free_percpu(new->scratch);
 	kfree(new);
 
-	return ERR_PTR(-ENOMEM);
+	return NULL;
 }
 
 /**
@@ -1797,7 +1797,7 @@ static void nft_pipapo_commit(struct nft_set *set)
 		return;
 
 	new_clone = pipapo_clone(priv->clone);
-	if (IS_ERR(new_clone))
+	if (!new_clone)
 		return;
 
 	priv->dirty = false;
@@ -1821,7 +1821,7 @@ static void nft_pipapo_abort(const struct nft_set *set)
 	m = rcu_dereference_protected(priv->match, nft_pipapo_transaction_mutex_held(set));
 
 	new_clone = pipapo_clone(m);
-	if (IS_ERR(new_clone))
+	if (!new_clone)
 		return;
 
 	priv->dirty = false;
@@ -2269,8 +2269,8 @@ static int nft_pipapo_init(const struct nft_set *set,
 
 	/* Create an initial clone of matching data for next insertion */
 	priv->clone = pipapo_clone(m);
-	if (IS_ERR(priv->clone)) {
-		err = PTR_ERR(priv->clone);
+	if (!priv->clone) {
+		err = -ENOMEM;
 		goto out_free;
 	}