LinuxKPI; cleanup slab.h a bit; move more free() into slab.c

Move kfree() into slab.c as an implementation and hide the private
function linux_kfree_async() entirely.

Remove a ; at the end of a define and sort some defines into place.

Remove extern from function declarations and move the closer to where
they belong.

Sort the functions into "base allocator/free" functions--these have
an implementation in slab.c and are ensuring contiguous physical
memory allocations.

Followed by inline functions using these base allocators to implement
their functionality; vmalloc/kvalloc, and misc functions.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D49572