batman-adv: initialize up/down values when adding a gateway

Without this initialization, gateways which actually announce up/down
bandwidth of 0/0 could be added. If these nodes get purged via
_batadv_purge_orig() later, the gw_node structure does not get removed
since batadv_gw_node_delete() updates the gw_node with up/down
bandwidth of 0/0, and the updating function then discards the change
and does not free gw_node.

This results in leaking the gw_node structures, which references other
structures: gw_node -> orig_node -> orig_node_ifinfo -> hardif. When
removing the interface later, the open reference on the hardif may cause
hangs with the infamous "unregister_netdevice: waiting for mesh1 to
become free. Usage count = 1" message.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>