ksched: correct return code for invalid priority

By convention, EINVAL is returned when validating arguments, not EPERM.
This matches the documented behaviour of sched_setscheduler(3), and that
of SCHED_OTHER.

PR: 227735
MFC after: 1 week
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D37021

(cherry picked from commit ba4782022ac840bd20f4e10cb95ca27d2742333a)