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

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
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D45815