batman-adv: do not silently ignore wrong condition

Only one neigh_node per orig_node should match a given
neighbor address, therefore, if more than one matching
neigh_node is found, a WARNING has to be triggered to let
the user know that something is wrong in the originator
state instead of silently skipping the error.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>