neigh: Add infrastructure for allocating device neigh privates.

netdev->neigh_priv_len records the private area length.

This will trigger for neigh_table objects which set tbl->entry_size
to zero, and the first instances of this will be forthcoming.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/atm/clip.c b/net/atm/clip.c
index 11439a7..aea7cad 100644
--- a/net/atm/clip.c
+++ b/net/atm/clip.c
@@ -535,6 +535,7 @@
 {
 	dev->netdev_ops = &clip_netdev_ops;
 	dev->type = ARPHRD_ATM;
+	dev->neigh_priv_len = sizeof(struct atmarp_entry);
 	dev->hard_header_len = RFC1483LLC_LEN;
 	dev->mtu = RFC1626_MTU;
 	dev->tx_queue_len = 100;	/* "normal" queue (packets) */