MD: Fix problem where hot-added drives are not resynced.

If a drive is added with HOT_ADD_DISK rather than ADD_NEW_DISK,
saved_raid_disk isn't initialised properly, and the drive can be
included in the array without a resync.

Signed-off-by: Adrian Bunk <bunk@stusta.de>

drivers/md/md.c

@@ -3258,6 +3258,7 @@ static int hot_add_disk(mddev_t * mddev, dev_t dev)
 	}
 	clear_bit(In_sync, &rdev->flags);
 	rdev->desc_nr = -1;
+	rdev->saved_raid_disk = -1;
 	err = bind_rdev_to_array(rdev, mddev);
 	if (err)
 		goto abort_export;