uml: use DIV_ROUND_UP

I just saw similar patches in the janitor kernel's list, and spotted place it
fits.
Signed-off-by: Jiri Olsa <olsajiri@gmail.com>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/ubd_kern.c

@@ -1178,8 +1178,8 @@ static void cowify_bitmap(__u64 io_offset, int length, unsigned long *cow_mask,
 	 * by one word.  Thanks to Lynn Kerby for the fix and James McMechan
 	 * for the original diagnosis.
 	 */
-	if(*cow_offset == ((bitmap_len + sizeof(unsigned long) - 1) /
-			   sizeof(unsigned long) - 1))
+	if (*cow_offset == (DIV_ROUND_UP(bitmap_len,
+					 sizeof(unsigned long)) - 1))
 		(*cow_offset)--;
 
 	bitmap_words[0] = bitmap[*cow_offset];