netfilter: nfnetlink_osf: fix using plain integer as NULL warning

Fixes the following sparse warning:

net/netfilter/nfnetlink_osf.c:274:24: warning:
 Using plain integer as NULL pointer
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_osf.c

@@ -271,7 +271,7 @@ const char *nf_osf_find(const struct sk_buff *skb,
 
 	tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts);
 	if (!tcp)
-		return false;
+		return NULL;
 
 	list_for_each_entry_rcu(kf, &nf_osf_fingers[ctx.df], finger_entry) {
 		f = &kf->finger;