Missing sched_priority bounds check in SCHED_OTHER + signed-overflow UB in p4prio_to_rtpprio

Summary

ksched_setscheduler validates sched_priority for RR/FIFO(:169-170) but NOT SCHED_OTHER(:181-184). p4prio_to_rtpprio(P)=RTP_PRIO_MAX-P=31-P. Extreme P -> signed overflow UB -> arbitrary u_short in lwp_rtprio.prio. NORMAL scheduler ignores prio currently. Latent.