As of r257209, all architectures have defined
VM_KMEM_SIZE_SCALE. In other words, every architecture is now
auto-sizing the kmem arena. This revision changes kmeminit() so
that the definition of VM_KMEM_SIZE_SCALE becomes mandatory and
the definition of VM_KMEM_SIZE becomes optional.
Replace or eliminate all existing definitions of VM_KMEM_SIZE.
With auto-sizing enabled, VM_KMEM_SIZE effectively became an
alternate spelling for VM_KMEM_SIZE_MIN on most architectures.
Use VM_KMEM_SIZE_MIN for clarity.
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