batman-adv: netlink: add routing_algo query

BATADV_CMD_GET_ROUTING_ALGOS is used to get the list of supported routing
algorithms.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[sven.eckelmann@open-mesh.com: Reduce the number of changes to
BATADV_CMD_GET_ROUTING_ALGOS, fix includes]
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>