get_balloon_scratch_page() disables preemption so we cannot call
alloc_page() in between get/put_balloon_scratch_page().  Shuffle bits
around in decrease_reservation() to avoid this.
Signed-off-by: NDavid Vrabel <david.vrabel@citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>