LU-685 obdclass: lu_object reclamation is inefficient

LU-685 obdclass: lu_object reclamation is inefficient

Put only non-referenced lu_object in lru list to speed up object
reclamation.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Signed-off-by: Bob Glossman <bogl@whamcloud.com>
Change-Id: Ibde905bfe7ec5ec0b66f31a6070081cf3dc331cd
Reviewed-on: http://review.whamcloud.com/2628
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>