linuxkpi: Move struct kobject code to linux_kobject.c

[Why]
linux_compat.c is already too long. I will need to add struct kset
in a follow-up commit, so let's move the existing struct kobject code
to its own file.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D43019

(cherry picked from commit 80446fc7b5e5d22e2bac28bc0474dbe2fec83e43)