linuxkpi: math.h: Add mul_u64_u32_div and mul_u64_u32_shr

Needed by drm-kmod.

Reviewed by: hselasky
Obtained from: OpenBSD
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35937

(cherry picked from commit 885ab0dba28b799c5c70923c2cb0ee162b5b75b1)