kernel: avoid overflow in cmp_range

Avoid overflow possibility.

[ The overflow is purely theoretical, since this is used for memory
  ranges that aren't even close to using the full 64 bits, but this is
  the right thing to do regardless.  - Linus ]
Signed-off-by: NLouis Langholtz <lou_langholtz@me.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Peter Anvin <hpa@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/range.c

@@ -113,12 +113,12 @@ static int cmp_range(const void *x1, const void *x2)
 {
 	const struct range *r1 = x1;
 	const struct range *r2 = x2;
-	s64 start1, start2;
 
-	start1 = r1->start;
-	start2 = r2->start;
-
-	return start1 - start2;
+	if (r1->start < r2->start)
+		return -1;
+	if (r1->start > r2->start)
+		return 1;
+	return 0;
 }
 
 int clean_sort_range(struct range *range, int az)