netfilter: nft_ct: fix expiration getter

We need to compute timeout.expires - jiffies, not the other way around.
Add a helper, another patch can then later change more places in
conntrack code where we currently open-code this.

Will allow us to only change one place later when we remove per-ct timer.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack.h

@@ -284,6 +284,14 @@ static inline bool nf_is_loopback_packet(const struct sk_buff *skb)
 	return skb->dev && skb->skb_iif && skb->dev->flags & IFF_LOOPBACK;
 }
 
+/* jiffies until ct expires, 0 if already expired */
+static inline unsigned long nf_ct_expires(const struct nf_conn *ct)
+{
+	long timeout = (long)ct->timeout.expires - (long)jiffies;
+
+	return timeout > 0 ? timeout : 0;
+}
+
 struct kernel_param;
 
 int nf_conntrack_set_hashsize(const char *val, struct kernel_param *kp);

net/netfilter/nft_ct.c

@@ -54,7 +54,6 @@ static void nft_ct_get_eval(const struct nft_expr *expr,
 	const struct nf_conn_help *help;
 	const struct nf_conntrack_tuple *tuple;
 	const struct nf_conntrack_helper *helper;
-	long diff;
 	unsigned int state;
 
 	ct = nf_ct_get(pkt->skb, &ctinfo);
@@ -94,10 +93,7 @@ static void nft_ct_get_eval(const struct nft_expr *expr,
 		return;
 #endif
 	case NFT_CT_EXPIRATION:
-		diff = (long)jiffies - (long)ct->timeout.expires;
-		if (diff < 0)
-			diff = 0;
-		*dest = jiffies_to_msecs(diff);
+		*dest = jiffies_to_msecs(nf_ct_expires(ct));
 		return;
 	case NFT_CT_HELPER:
 		if (ct->master == NULL)