Replace kernel virtual address space allocation with vmem. This provides
transparent layering and better fragmentation.

• Normalize functions that allocate memory to use kmem_*
• Those that allocate address space are named kva_*
• Those that operate on maps are named kmap_*
• Implement recursive allocation handling for kmem_arena in vmem.

Reviewed by: alc
Tested by: pho
Sponsored by: EMC / Isilon Storage Division