batman-adv: fix neigh reference imbalance

When an interface is removed from batman-adv, the orig_ifinfo of a
orig_node may be removed without releasing the router first.
This will prevent the reference for the neighbor pointed at by the
orig_ifinfo->router to be released, and this leak may result in
reference leaks for the interface used by this neighbor. Fix that.

This is a regression introduced by
7351a482
("batman-adv: split out router from orig_node").

Reported-by: Antonio Quartulli <antonio@open-mesh.com>
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>