staging: lustre: nrs: serialize executions of nrs_policy_stop

staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7096
Reviewed-on: http://review.whamcloud.com/16214
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>