proc: expose dirty/writeback accounting

[0xzre]

Resolves #203

• Wire meminfo Dirty/Writeback fields to the dirty/writeback accounting.
• Add global Dirty/Writeback counters in usage and hook DirtySet ops (mark/clean/sync/remove) to update them.
• Adjust gofer cleanup paths to use accounted removal
• Add a unit test for dirty accounting.

Note

with directfs/InteropModeShared defaults, writes bypass the sandbox cache, so values often remain 0