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).

[bz]

Anyone?

[bz]

If no one replies this will go in on Monday.

[emaste]

OK

[bz]

Updating for reference after rebase; also changed linuxkpi_ to lkpi_
as per recent discussions until we have a README.

[imp]

I have a stale review comment somewhere I can't find now. But it said something like "I like this change because we do way too much inlining in the linuxkpi because that causes problems with our goal to have a stable ABI on a stable branch"

[emaste]

In D45815#1052042, @imp wrote:

"I like this change because we do way too much inlining in the linuxkpi because that causes problems with our goal to have a stable ABI on a stable branch"

Yeah I'd say any potential performance gain from inlining it pales in comparison to the maintainability improvement from not doing so.