batman-adv: protect softif_neigh by rcu

Add get/set wrapper functions for softif_neigh and
use rcu functions to manipulate the pointers.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>