[PATCH] Fix off by one error in page_region_mask()

[Lachlan McIlroy]

final is calculated to be the last bit to set (ie inclusive) but when we
do the mask shifting final really needs to be first bit not to set.

For example if first and final are both bit 0 (ie only first bit to be set)
then mask is completely shifted and becomes all zeroes.

Or if first is 0 and final is 63 then the mask is shifted one bit when it
shouldn't be shifted at all.

--- xfs-fix.orig/fs/xfs/linux-2.6/xfs_buf.c
+++ xfs-fix/fs/xfs/linux-2.6/xfs_buf.c
@@ -129,15 +129,17 @@ page_region_mask(
  	int		first, final;

  	first = BTOPR(offset);
-	final = BTOPRT(offset + length - 1);
-	first = min(first, final);
+	final = BTOPRT(offset + length);
+
+	if (first >= final)
+		return 0UL;

  	mask = ~0UL;
  	mask <<= BITS_PER_LONG - (final - first);
  	mask >>= BITS_PER_LONG - (final);

  	ASSERT(offset + length <= PAGE_CACHE_SIZE);
-	ASSERT((final - first) < BITS_PER_LONG && (final - first) >= 0);
+	ASSERT((final - first) <= BITS_PER_LONG && (final - first) > 0);

  	return mask;
  }