ztress: fix the progress stats

Current ztress design supports both timer context and thread
context. The timer context always uses the first element of
several global array variables like exec_cnt[] and backoff[].
But progress_timeout() is using the last element of exec_cnt[]
to calculate the progress of timer context. It is not right.

Signed-off-by: Ming Shao <ming.shao@intel.com>