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>

include/linux/netdevice.h

@@ -1080,6 +1080,7 @@ struct net_device {
 	unsigned char		perm_addr[MAX_ADDR_LEN]; /* permanent hw address */
 	unsigned char		addr_assign_type; /* hw address assignment type */
 	unsigned char		addr_len;	/* hardware address length	*/
+	unsigned char		neigh_priv_len;
 	unsigned short          dev_id;		/* for shared network cards */
 
 	spinlock_t		addr_list_lock;