batman-adv: move neigh_node list add into batadv_neigh_node_new()

All batadv_neigh_node_* functions expect the neigh_node list item to be part
of the orig_node->neigh_list, therefore the constructor of said list item
should be adding the newly created neigh_node to the respective list.

Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Acked-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>