LU-7096 nrs: serialize executions of nrs_policy_stop

LU-7096 nrs: serialize executions of nrs_policy_stop

Do not release nrs_lock in nrs_policy_stop0 to prevent op_policy_stop()
from being executed concurrently.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: Ie42793021aa47ff7e2c14eb58b3d6e8405fa8407
Reviewed-on: http://review.whamcloud.com/16214
Tested-by: Jenkins
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>