Fix: fencing: Correctly track active stonith actions

When executing a stonith action, stonith_action_async_done() can
re-invoke internal_stonith_action_execute(). In that case, it will
introduce a new pid for the action, which can not be tracked by the
invoker of stonith_action_execute_async().

This commit fixes it by adding stonith_device_t *active_on to
async_command_t.