Use P1B_PRIO_MAX to designate max posix priority for the RR/FIFO

scheduler types.  It was intended to be used there, compare with the
min value, and with the test for correctness in ksched_setscheduler().

Note that P1B_PRIO_MAX and RTP_PRIO_MAX do have the same numerical
values, the change is cosmetical.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/ksched.c

@@ -229,7 +229,7 @@ ksched_get_priority_max(struct ksched *ksched, int policy, int *prio)
 	switch (policy)	{
 	case SCHED_FIFO:
 	case SCHED_RR:
-		*prio = RTP_PRIO_MAX;
+		*prio = P1B_PRIO_MAX;
 		break;
 	case SCHED_OTHER:
 		*prio = PRI_MAX_TIMESHARE - PRI_MIN_TIMESHARE;