netfilter: nft_set_pipapo: skip inactive elements during set walk

Otherwise set elements can be deactivated twice which will cause a crash.

Reported-by: Xingyuan Mo <hdthky0@gmail.com>
Fixes: 3c4287f ("nf_tables: Add set type for arbitrary concatenation of ranges")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_set_pipapo.c

@@ -2043,6 +2043,9 @@ static void nft_pipapo_walk(const struct nft_ctx *ctx, struct nft_set *set,
 
 		e = f->mt[r].e;
 
+		if (!nft_set_elem_active(&e->ext, iter->genmask))
+			goto cont;
+
 		iter->err = iter->fn(ctx, set, iter, &e->priv);
 		if (iter->err < 0)
 			goto out;