acpi, nfit, libnvdimm: fix interleave set cookie calculation (64-bit comparison)

While reviewing the -stable patch for commit 86ef58a4e35e "nfit,
libnvdimm: fix interleave set cookie calculation" Ben noted:

    "This is returning an int, thus it's effectively doing a 32-bit
     comparison and not the 64-bit comparison you say is needed."

Update the compare operation to be immune to this integer demotion problem.

Cc: <stable@vger.kernel.org>
Cc: Nicholas Moulin <nicholas.w.moulin@linux.intel.com>
Fixes: 86ef58a4e35e ("nfit, libnvdimm: fix interleave set cookie calculation")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit/core.c

@@ -1617,7 +1617,11 @@ static int cmp_map(const void *m0, const void *m1)
 	const struct nfit_set_info_map *map0 = m0;
 	const struct nfit_set_info_map *map1 = m1;
 
-	return map0->region_offset - map1->region_offset;
+	if (map0->region_offset < map1->region_offset)
+		return -1;
+	else if (map0->region_offset > map1->region_offset)
+		return 1;
+	return 0;
 }
 
 /* Retrieve the nth entry referencing this spa */