LinuxKPI: Move linux/math.h content out of linux/kernel.h

To match Linux and allow direct linux/math.h inclusion.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D45452

(cherry picked from commit 588064e658d32f037854dd852b988728197a28de)