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>

include/uapi/linux/if_link.h

@@ -161,6 +161,8 @@ enum {
 	IFLA_EVENT,
 	IFLA_NEW_NETNSID,
 	IFLA_IF_NETNSID,
+	IFLA_CARRIER_UP_COUNT,
+	IFLA_CARRIER_DOWN_COUNT,
 	__IFLA_MAX
 };