[INET]: Fix netdev renaming and inet address labels

When re-naming an interface, the previous secondary address
labels get lost e.g.

  $> brctl addbr foo
  $> ip addr add 192.168.0.1 dev foo
  $> ip addr add 192.168.0.2 dev foo label foo:00
  $> ip addr show dev foo | grep inet
    inet 192.168.0.1/32 scope global foo
    inet 192.168.0.2/32 scope global foo:00
  $> ip link set foo name bar
  $> ip addr show dev bar | grep inet
    inet 192.168.0.1/32 scope global bar
    inet 192.168.0.2/32 scope global bar:2

Turns out to be a simple thinko in inetdev_changename() - clearly we
want to look at the address label, rather than the device name, for
a suffix to retain.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/devinet.c

@@ -1027,7 +1027,7 @@ static void inetdev_changename(struct net_device *dev, struct in_device *in_dev)
 		memcpy(ifa->ifa_label, dev->name, IFNAMSIZ);
 		if (named++ == 0)
 			continue;
-		dot = strchr(ifa->ifa_label, ':');
+		dot = strchr(old, ':');
 		if (dot == NULL) {
 			sprintf(old, ":%d", named);
 			dot = old;