LinuxKPI: move __kmalloc from slab.h to slab.c

[bz]

Summary

In order to allow the allocator to change in the future move it into
the implementation file from being an inline function in the header.

While here factor out the size calculation and add a comment as-to why
this is done. We will need the size (_s) in the future to make a
decision on how to allocate.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

Test Plan

(I amnually extracted this modulo the extra changes on top
of D45812 as moving it can go independetly upfront).