Refactor existing code

Refactor existing code

This change introduces no functional changes to the memory management
interfaces.  It only restructures the existing codes by separating the
kmem, vmem, and kmem cache implementations in the separate source and
header files.

Splitting this functionality in to separate files required the addition
of spl_vmem_{init,fini}() and spl_kmem_cache_{initi,fini}() functions.

Additionally, several minor changes to the #include's were required to
accommodate the removal of extraneous header from kmem.h.

But again, while large this patch introduces no functional changes.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>