LinuxKPI: Remove linux/sched.h include from linux/kernel.h header

This include prevents usage of any kernel.h helpers in sched.h and
all of dependencies. Linux does not have it too.
Fix building of kernel and drm-kmod after than.

Sponsored by: Serenity CyberSecurity, LLC
MFC after: 1 week
Reviewed by: manu, bz
Differential Revision: https://reviews.freebsd.org/D45692

(cherry picked from commit 256eb8d5361143b7d436a195530e0eefe1450e6d)