netfilter: nft_last: incorrect arithmetics when restoring last used

Subtract the jiffies that have passed by to current jiffies to fix last
used restoration.

Fixes: 836382d ("netfilter: nf_tables: add last expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_last.c

@@ -37,7 +37,7 @@ static int nft_last_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		if (err < 0)
 			return err;
 
-		priv->last_jiffies = jiffies + (unsigned long)last_jiffies;
+		priv->last_jiffies = jiffies - (unsigned long)last_jiffies;
 	}
 
 	return 0;