linuxkpi: Add sched/mm.h
With stubs needed by drm-kmod.
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36100
(cherry picked from commit 8828ebd6fc406520273e59807e66e24eae654f71)