Implement the Free Memory Fragmentation Index (FMFI) metric

[bojan.novkovic_fer.hr]

Summary

This patch adds the Free Memory Fragmentation Index (FMFI) metric to the vm_phys subsystem.

The FMFI metric measures the degree of physical memory fragmentation for a given order by using metadata from the buddy allocator freelists.
Its value ranges from arbitrary negative values up to 1. A negative value implies that there is ample memory to serve an allocation request of the given order.
A value between 0 (no fragmentation) and 1 (highly fragmented) indicates the degree of physical memory fragmentation.

This work was sponsored by the Google Summer of Code program.

Test Plan

This metric is regularly used in academic papers regarding superpage management and is also used in the Linux kernel's memory compaction subsystem.
More information on this metric is available at the following link.

[bojan.novkovic_fer.hr]

Handle possible divide-by-zero exception when info.free_blocks is 0.