wqueue: improve the robustness of the work

struct work_s
{
  union
  {
    struct
    {
      struct dq_entry_s dq;      /* Implements a double linked list */
      clock_t qtime;             /* Time work queued */
    } s;
    struct wdog_s timer;         /* Delay expiry timer */
    struct wdog_period_s ptimer; /* Period expiry timer */
  } u;
  worker_t  worker;              /* Work callback */
  FAR void *arg;                 /* Callback argument */
  FAR struct kwork_wqueue_s *wq; /* Work queue */
};

work_cancel() should determine whether the current work is
in the timer or has already entered the queue.
This judgment is indispensable because the structure is a union.
Whether it is interpreted as a timer or as a dq needs to be determined.

But this judgment seriously depends on the order of struct wdog_s and
struct dq_entry_s, once someone change the order of any, there is a bug.
So we decide remove the union, to improve the robustness.

For the work_s structure size will grow bigger, then we will provide a
another optimization patch

Signed-off-by: ligd <liguiding1@xiaomi.com>
Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

libs/libc/wqueue/work_usrthread.c

@@ -125,7 +125,7 @@ static void work_process(FAR struct usr_wqueue_s *wqueue)
        * zero will always execute immediately.
        */
 
-      elapsed = clock() - work->u.s.qtime;
+      elapsed = clock() - work->qtime;
 
       /* Is this delay work ready? */
 
@@ -180,7 +180,7 @@ static void work_process(FAR struct usr_wqueue_s *wqueue)
         }
       else
         {
-          next = work->u.s.qtime - clock();
+          next = work->qtime - clock();
           break;
         }
     }