linuxkpi: Add kfifo.h
Provide macros needed for drm-kmod.
Reviewed by: bz
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35563
(cherry picked from commit 088b746cbda664602f9cee0d1415ae8d98389f38)