md: update comment for md_allow_write

MD_CHANGE_CLEAN had been replaced with MD_CHANGE_PENDING after
commit 070dc6 ("md: resolve confusion of MD_CHANGE_CLEAN"),
so make the change accordingly.
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/md.c

@@ -7714,7 +7714,7 @@ EXPORT_SYMBOL(md_write_end);
  * attempting a GFP_KERNEL allocation while holding the mddev lock.
  * Must be called with mddev_lock held.
  *
- * In the ->external case MD_CHANGE_CLEAN can not be cleared until mddev->lock
+ * In the ->external case MD_CHANGE_PENDING can not be cleared until mddev->lock
  * is dropped, so return -EAGAIN after notifying userspace.
  */
 int md_allow_write(struct mddev *mddev)