netfilter: nft_set_pipapo: don't return bogus extension pointer

Dan Carpenter says:
Commit 17a20e0 ("netfilter: nft_set: remove one argument from
lookup and update functions") [..] leads to the following Smatch
static checker warning:

 net/netfilter/nft_set_pipapo_avx2.c:1269 nft_pipapo_avx2_lookup()
 error: uninitialized symbol 'ext'.

Fix this by initing ext to NULL and set it only once we've found
a match.

Fixes: 17a20e0 ("netfilter: nft_set: remove one argument from lookup and update functions")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/netfilter-devel/aJBzc3V5wk-yPOnH@stanley.mountain/
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: Florian Westphal

net/netfilter/nft_set_pipapo_avx2.c

@@ -1150,12 +1150,12 @@ nft_pipapo_avx2_lookup(const struct net *net, const struct nft_set *set,
 		       const u32 *key)
 {
 	struct nft_pipapo *priv = nft_set_priv(set);
+	const struct nft_set_ext *ext = NULL;
 	struct nft_pipapo_scratch *scratch;
 	u8 genmask = nft_genmask_cur(net);
 	const struct nft_pipapo_match *m;
 	const struct nft_pipapo_field *f;
 	const u8 *rp = (const u8 *)key;
-	const struct nft_set_ext *ext;
 	unsigned long *res, *fill;
 	bool map_index;
 	int i;
@@ -1246,13 +1246,13 @@ nft_pipapo_avx2_lookup(const struct net *net, const struct nft_set *set,
 			goto out;
 
 		if (last) {
-			ext = &f->mt[ret].e->ext;
-			if (unlikely(nft_set_elem_expired(ext) ||
-				     !nft_set_elem_active(ext, genmask))) {
-				ext = NULL;
+			const struct nft_set_ext *e = &f->mt[ret].e->ext;
+
+			if (unlikely(nft_set_elem_expired(e) ||
+				     !nft_set_elem_active(e, genmask)))
 				goto next_match;
-			}
 
+			ext = e;
 			goto out;
 		}