net: Make ndo_neigh_destroy return void.

The return value isn't used.

Suggested by Ben Hucthings.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -975,7 +975,7 @@ struct net_device_ops {
 	int			(*ndo_set_features)(struct net_device *dev,
 						    netdev_features_t features);
 	int			(*ndo_neigh_construct)(struct neighbour *n);
-	int			(*ndo_neigh_destroy)(struct neighbour *n);
+	void			(*ndo_neigh_destroy)(struct neighbour *n);
 };
 
 /*