batman-adv: move hardif refcount inc to batadv_neigh_node_new()

The batadv_neigh_node cleanup function 'batadv_neigh_node_free_rcu()'
takes care of reducing the hardif refcounter, hence it's only logical
to assume the creating function of that same object
'batadv_neigh_node_new()' takes care of increasing the same refcounter.

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