netfilter: nf_tables: fix error return code

Convert a zero return value on error to a negative one, as returned
elsewhere in the function.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
identifier ret; expression e1,e2;
@@
(
if (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -3137,8 +3137,10 @@ static int nft_del_setelem(struct nft_ctx *ctx, struct nft_set *set,
 		goto err2;
 
 	trans = nft_trans_elem_alloc(ctx, NFT_MSG_DELSETELEM, set);
-	if (trans == NULL)
+	if (trans == NULL) {
+		err = -ENOMEM;
 		goto err2;
+	}
 
 	nft_trans_elem(trans) = elem;
 	list_add_tail(&trans->list, &ctx->net->nft.commit_list);