net: core: Expose number of link up/down transitions

Expose the number of times the link has been going UP or DOWN, and
update the "carrier_changes" counter to be the sum of these two events.
While at it, also update the sysfs-class-net documentation to cover:
carrier_changes (3.15), carrier_up_count (4.16) and carrier_down_count
(4.16)

Signed-off-by: David Decotigny <decot@googlers.com>
[Florian:
* rebase
* add documentation
* merge carrier_changes with up/down counters]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index ef8b4ec..1816bde 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -510,7 +510,7 @@ void netif_carrier_on(struct net_device *dev)
 	if (test_and_clear_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
 		if (dev->reg_state == NETREG_UNINITIALIZED)
 			return;
-		atomic_inc(&dev->carrier_changes);
+		atomic_inc(&dev->carrier_up_count);
 		linkwatch_fire_event(dev);
 		if (netif_running(dev))
 			__netdev_watchdog_up(dev);
@@ -529,7 +529,7 @@ void netif_carrier_off(struct net_device *dev)
 	if (!test_and_set_bit(__LINK_STATE_NOCARRIER, &dev->state)) {
 		if (dev->reg_state == NETREG_UNINITIALIZED)
 			return;
-		atomic_inc(&dev->carrier_changes);
+		atomic_inc(&dev->carrier_down_count);
 		linkwatch_fire_event(dev);
 	}
 }