net: linkwatch: allow vlans to get carrier changes faster

There is a time-lag of IFF_RUNNING flag consistency between vlan and
real devices when the real devices are in problem such as link or cable
broken.

This leads to a degradation of Availability such as a delay of failover
in HA systems using vlan since the detection of the problem at real
device is delayed.

We can avoid the linkwatch delay (~1 sec) for devices linked to another
ones, since delay is already done for the realdev.

Based on a previous patch from Mitsuo Hayasaka

Reported-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Patrick McHardy <kaber@trash.net>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Tom Herbert <therbert@google.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Jesse Gross <jesse@nicira.com>
Tested-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/link_watch.c

@@ -78,8 +78,13 @@ static void rfc2863_policy(struct net_device *dev)
 
 static bool linkwatch_urgent_event(struct net_device *dev)
 {
-	return netif_running(dev) && netif_carrier_ok(dev) &&
-		qdisc_tx_changing(dev);
+	if (!netif_running(dev))
+		return false;
+
+	if (dev->ifindex != dev->iflink)
+		return true;
+
+	return netif_carrier_ok(dev) &&	qdisc_tx_changing(dev);
 }