be_ptask: backoff not applied on first re-schedule

The task interval backoff is not applied on the first re-schedule
operation, since when scheduling the first run (BE_PTASK_FIRST_DELAY)
we do not calculate the backed off period for the next re-schedule.

Calculate the backed off period for the current scheduling operation,
rather than the next, to resolve this.

src/providers/be_ptask.c

@@ -214,12 +214,12 @@ static void be_ptask_schedule(struct be_ptask *task,
             return;
         }
 
-        delay = task->period;
-
         if (backoff_allowed(task)) {
             /* double the period for the next execution */
             task->period = MIN(task->period * 2, task->max_backoff);
         }
+
+        delay = task->period;
         break;
     }