Implicit undocumented locking contract on lwp_rtprio writes

Summary

ksched_setscheduler writes lp->lwp_rtprio(:175,:184) with no lock/assert. Correctness depends on caller holding lwp_token(kern_p1003_1b.c:208,234,264). Contract undocumented. Future caller missing token -> torn write -> inconsistent type/prio.