net: fib: Decrease one unnecessary rt cache flush in fib_disable_ip

The func fib_flush already flushes the rt cache if necessary, so it
is not necessary to invoke rt_cache_flush again in fib_disable_ip.

Signed-off-by: Gao Feng <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/fib_frontend.c

···
 {
 	if (fib_sync_down_dev(dev, event, force))
 		fib_flush(dev_net(dev));
-	rt_cache_flush(dev_net(dev));
+	else
+		rt_cache_flush(dev_net(dev));
 	arp_ifdown(dev);
 }