Restore partition detection of newly created md arrays.

Commit  b821eaa572fd737faaf6928ba046e571526c36c6 broke partition
detection for md arrays.

The logic was almost right.  However if revalidate_disk is called
when the device is not yet open, bdev->bd_disk won't be set, so the
flush_disk() Call will not set bd_invalidated.

So when md_open is called we still need to ensure that
->bd_invalidated gets set.  This is easily done with a call to
check_disk_size_change in the place where the offending commit removed
check_disk_change.  At the important times, the size will have changed
from 0 to non-zero, so check_disk_size_change will set bd_invalidated.

Tested-by: Duncan <1i5t5.duncan@cox.net>
Reported-by: Duncan <1i5t5.duncan@cox.net>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -5895,6 +5895,7 @@ static int md_open(struct block_device *bdev, fmode_t mode)
 	atomic_inc(&mddev->openers);
 	mutex_unlock(&mddev->open_mutex);
 
+	check_disk_size_change(mddev->gendisk, bdev);
  out:
 	return err;
 }