[$] Weighted interleaving for memory tiering

The kernel has, for many years, had the ability to control how memory
allocation is performed in systems with multiple NUMA nodes. More
recently, NUMA nodes have also been pressed into service to represent
different classes of memory; those nodes are now organized into tiers
according to their performance characteristics. While memory-allocation
policies can control the placement of pages at the NUMA-node level, the
kernel provides no way to connect those policies with memory tiers. This
patch series from Gregory Price aims to change this situation by
allowing allocations to be placed across tiers in a weighted manner.