hv_balloon: simplify math in alloc_balloon_pages()

'alloc_unit' in alloc_balloon_pages() is either '512' for 2M allocations or
'1' for 4k allocations. So

1 << get_order(alloc_unit << PAGE_SHIFT)

equals to 'alloc_unit' and the for loop basically sets all them offline.
Simplify the math to improve the readability.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201202161245.2406143-2-vkuznets@redhat.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>