Improve handling of spares in md

- hot_remove_disk is given the raid_disk rather than descriptor number
  so that it can find the device in internal array directly, no search.
- spare_inactive now uses mddev->spare->raid_disk instead of
  mddev->spare->number so it can find the device directly without searching
- spare_write does not need number.  It can use mddev->spare->raid_disk as above.
- spare_active does not need &mddev->spare.  It finds the descriptor directly
  and fixes it without this pointer