net: if_arp: use define instead of hard-coded value

uapi/linux/if_arp.h includes linux/netdevice.h, which uses
IFNAMSIZ. Hence, use it instead of hard-coded value.
Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_arp.h

@@ -118,7 +118,7 @@ struct arpreq {
 	struct sockaddr	arp_ha;		/* hardware address		 */
 	int		arp_flags;	/* flags			 */
 	struct sockaddr arp_netmask;    /* netmask (only for proxy arps) */
-	char		arp_dev[16];
+	char		arp_dev[IFNAMSIZ];
 };
 
 struct arpreq_old {