fix can not clean the last abandoned snat table (#2701)

Author: changluyi

pkg/daemon/gateway_linux.go

@@ -378,7 +378,7 @@ func (c *Controller) updateIptablesChain(protocol, table, chain, parent string,
 		added++
 	}
 	for i := len(existingRules) - 1; i >= len(rules)-added; i-- {
-		if err = c.iptables[protocol].Delete(table, chain, strconv.Itoa(i+added)); err != nil {
+		if err = c.iptables[protocol].Delete(table, chain, strconv.Itoa(i+added+1)); err != nil {
 			klog.Errorf(`failed to delete iptables rule %v: %v`, existingRules[i], err)
 			return err
 		}