Negative ARP header length are not a thing.

Constify arguments while I'm at it.

Space savings:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-3 (-3)
	function                        old     new   delta
	arpt_do_table                  1163    1160      -3
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>