linuxkpi: move kobject_create to .c file

kobject_create knows the size of struct kobject. Move it to
linux_compat.c so this knowledge is confined to the loadable module and
not the clients.

Sponsored by: Netflix
Reviewed by: hselasky, emaste
Differential Revision: https://reviews.freebsd.org/D34767

(cherry picked from commit 36929b55849c8f553a744d6e4fbc8b1a5ff73f66)