Fix calculation for size of filemap_attr array in md/bitmap.

If 'num_pages' were ever 1 more than a multiple of 8 (32bit platforms)
for of 16 (64 bit platforms). filemap_attr would be allocated one
'unsigned long' shorter than required.  We need a round-up in there.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/md/bitmap.c

@@ -863,9 +863,7 @@ static int bitmap_init_from_disk(struct bitmap *bitmap, sector_t start)
 
 	/* We need 4 bits per page, rounded up to a multiple of sizeof(unsigned long) */
 	bitmap->filemap_attr = kzalloc(
-		(((num_pages*4/8)+sizeof(unsigned long)-1)
-		 /sizeof(unsigned long))
-		*sizeof(unsigned long),
+		roundup( DIV_ROUND_UP(num_pages*4, 8), sizeof(unsigned long)),
 		GFP_KERNEL);
 	if (!bitmap->filemap_attr)
 		goto out;