batman-adv: add B.A.T.M.A.N. IV bat_gw_dump implementations

Dump the list of gateways via the netlink socket.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[sven.eckelmann@open-mesh.com: integrate in batadv_algo_ops]
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>