linuxkpi: Move class_create to .c file

class_create encodes the size of struct class into the generated
code. Move from .h file to .c file to move this knowledge from the
client modules that call this into the linuxkpi module.

Sponsored by: Netflix
Reviewed by: hselasky, emaste
Differential Revision: https://reviews.freebsd.org/D34769

(cherry picked from commit 1341ac9f9c111bba4e1ca046c479f32f2d4989c0)