stress plugin: sm_read_number is never updated for threads

[LiXi-storage]

See envzhu/collectd@13d3547

To fix the problem of not updating sm_read_number for each thread, instead of changing stress_metrics of all threads, I'd recommend to do the following change:

1. add "int *sm_read_number_pointer;" in "struct stress_metric"
   2.. in stress_setup_environment_thread(), add "stress_metrics[metric_index].sm_read_number_pointer = &(stress_metric->sm_read_number);"
2. In stress_variable_field_get(), change "int read_number = sm->sm_read_number;" to "int read_number = *sm->sm_read_number_pointer ;"
3. Other places remain the same. We don't care the value of sm_read_number_pointer in se_metric_head, and we don't care the value of sm_read_number in se_thread_datas[i].std_stress_metrics