LinuxKPI: Add abs_diff to linux/math.h

It returns absolute value of the difference between the arguments.

Add LinuxKPI version check as the macro was moved from drm-kmod to
linux/math.h in Linux kernel commit 46f12960aad (6.6 timeframe).

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz, emaste
Differential Revision: https://reviews.freebsd.org/D45453