If we currently change the mac address inside the wpan interface while
we have a lowpan interface on top of the wpan interface, the mac address
setting doesn't reach the lowpan interface. The effect would be that the
IPv6 lowpan interface has the old SLAAC address and isn't working
anymore because the lowpan interface use in internal mechanism sometimes
dev->addr which is the old mac address of the wpan interface.

This patch checks if a wpan interface belongs to lowpan interface, if
yes then we need to check if the lowpan interface is down and change the
mac address also at the lowpan interface. When the lowpan interface will
be set up afterwards, it will use the correct SLAAC address which based
on the updated mac address setting.
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Tested-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>