linuxkpi: Add smp_mb__before/after_atomic
Reviewed by: hselasky
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36020
(cherry picked from commit eca2f0f3806647f165162e6db35d1af9aba410fa)