wqueue: fix work_qcancel() judge error caused by the union in struct work_s

in 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 */
}

while we use WDOG_ISACTIVE(&work->timer) to decide use dq or timer,
that is error, wd_cancel() maybe modify this area, dq_rem() also can
modify this area.
So we can't use the WDOG_ISACTIVE(&work->timer) to take as the judgement,
when there is a union.

Fix:
swap the order in struct wdog_s, move the arg to the second 32bit

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

include/nuttx/wdog.h

@@ -61,17 +61,22 @@ typedef uint32_t  wdparm_t;
 
 typedef CODE void (*wdentry_t)(wdparm_t arg);
 
-/* This is the internal representation of the watchdog timer structure. */
+/* This is the internal representation of the watchdog timer structure.
+ * Notice !!!
+ * Carefully with the struct wdog_s order, you may not directly modify
+ * this. This struct will combine in struct work_s in union type, and,
+ * wqueue will modify/check this struct in kwork work_qcancel().
+ */
 
 struct wdog_s
 {
   FAR struct wdog_s *next;       /* Support for singly linked lists. */
+  wdparm_t           arg;        /* Callback argument */
   wdentry_t          func;       /* Function to execute when delay expires */
 #ifdef CONFIG_PIC
   FAR void          *picbase;    /* PIC base address */
 #endif
   sclock_t           lag;        /* Timer associated with the delay */
-  wdparm_t           arg;        /* Callback argument */
 };
 
 /****************************************************************************