LU-3059 obdclass: use a dummy structure for lu_ref_link

LU-3059 obdclass: use a dummy structure for lu_ref_link

Move the definition of struct lu_ref_link to lu_ref.h.  If USE_LU_REF
is not defined then define it to be the empty struct.  Change the
return type of lu_ref_add() and lu_ref_add_atomic() to void.  Add
lu_ref_add_at() taking same arguments as lu_ref_add() togerther with a
pointer to a struct lu_ref_link and returning void.  Adjust all
structures containing a lu_ref_link pointer to contain a struct
lu_ref_link instead.  Use lu_ref_add_at() and lu_ref_del_at() to
handle embedded lu_ref_links.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I9d35b0dded24720dc332828b49d0b0a1cd51f620
Reviewed-on: http://review.whamcloud.com/5920
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>