LU-5397 obdclass: optimize busy loop wait

LU-5397 obdclass: optimize busy loop wait

two optimizations in the busy loop wait in class_cleanup():
(1) remove unnecessary spinlocks protecting obd_conn_inprogress
    access
(2) replace cond_resched by more suitable yield to unconditionally
    yield the CPU if the condition check fails.

Xyratex-bug-id: MRP-1991
Signed-off-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Change-Id: I9079b2d04afda7d6db902392c6dcb216706a7ae4
Reviewed-on: http://review.whamcloud.com/11198
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>