batman-adv: Modify neigh_list only with rcu-list functions

The batadv_hard_iface::neigh_list is accessed via rcu based primitives.
Thus all operations done on it have to fulfill the requirements by RCU. So
using non-RCU mechanisms like hlist_add_head is not allowed because it
misses the barriers required to protect concurrent readers when accessing
the data behind the pointer.

Fixes: cef63419 ("batman-adv: add list of unique single hop neighbors per hard-interface")
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>