PM / hibernate: Avoid overflow in hibernate_preallocate_memory()

PM / hibernate: Avoid overflow in hibernate_preallocate_memory()

When system has a lot of highmem (e.g. 16GiB using a 32 bits kernel),
the code to calculate how much memory we need to preallocate in
normal zone may cause overflow. As Leon has analysed:

 It looks that during computing 'alloc' variable there is overflow:
 alloc = (3943404 - 1970542) - 1978280 = -5418 (signed)
 And this function goes to err_out.

Fix this by avoiding that overflow.

References: https://bugzilla.kernel.org/show_bug.cgi?id=60817
Reported-and-tested-by: Leon Drugi <eyak@wp.pl>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>