Fix an off-by-one error in r228960.  The maximum priority delta provided
by SCHED_PRI_TICKS should be SCHED_PRI_RANGE - 1 so that the resulting
priority value (before nice adjustment) is between SCHED_PRI_MIN and
SCHED_PRI_MAX, inclusive.

Submitted by:	kib
Reported by:	pho
MFC after:	1 week