LU-2543 llite: wait osc cleanup when put super

LU-2543 llite: wait osc cleanup when put super

Since osc is now accessing sbi->ll_cache, we'd wait till all OSCs
are gone when put super.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I5d09b29cda28d4b596f67bdf8da2ce5e7082a38e
Reviewed-on: http://review.whamcloud.com/4951
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>