batman-adv: free neighbors when an interface is deactivated

hardif_disable_interface() calls purge_orig_ref() to immediately free
all neighbors associated with the interface that is going down.
purge_orig_neighbors() checked if the interface status is IF_INACTIVE
which is set to IF_NOT_IN_USE shortly before calling purge_orig_ref().

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>