- Cast the intermediate value in priority computtion back down to

unsigned char.  Weirdly, casting the 1 constant to u_char still produces
   a signed integer result that is then used in the % computation.  This
   avoids that mess all together and causes a 0 pri to turn into 255 % 64
   as we expect.

Reported by:	kkenn (about 4 times, thanks)

sys/kern/sched_ule.c

@@ -388,7 +388,7 @@ tdq_runq_add(struct tdq *tdq, struct td_sched *ts, int flags)
 			 */
 			if (tdq->tdq_ridx != tdq->tdq_idx &&
 			    pri == tdq->tdq_ridx)
-				pri = (pri - 1) % RQ_NQS;
+				pri = (unsigned char)(pri - 1) % RQ_NQS;
 		} else
 			pri = tdq->tdq_ridx;
 		runq_add_pri(ts->ts_runq, ts, pri, flags);