- introduce open counter in mdd

- introduce open counter in mdd
- update mo_close() method with adding the md_attr
- split mdd_attr_get() into two sub-function __mdd_atte_get, __mdd_lov_get
  because they are needed for internal use in mdd
- return lov ea and llog cookie in mdt_close() for unlinked file
- mdt_handle_last_unlink() checks the ma_valid fields to decide what to return
  to the client