batman-adv: Use rtnl link in device creation example

The standard kernel API to add new virtual interfaces and attach other
interfaces to it is rtnl-link. batman-adv supports it since v3.10. This
functionality should be used instead of the legacy batman-adv-only sysfs
interface.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>