linuxkpi: Add compat_ptr and ptr_to_compat
Needed by drm-kmod.
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36016
(cherry picked from commit 37cda2837c004e743e9262e2e0ea409766df2a09)