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>