net: use 32-bit arithmetic while allocating net device

Private part of allocation is never big enough to warrant size_t.

Space savings:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-10 (-10)
	function                                     old     new   delta
	alloc_netdev_mqs                            1120    1110     -10

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>