Kernel+SystemMonitor: Expose amount of per-process dirty private memory

Dirty private memory is all memory in non-inode-backed mappings that's
process-private, meaning it's not shared with any other process.

This patch exposes that number via SystemMonitor, giving us an idea of
how much memory each process is responsible for all on its own.

Kernel/VM/Region.h

@@ -103,6 +103,7 @@ public:
 
     size_t amount_resident() const;
     size_t amount_shared() const;
+    size_t amount_dirty() const;
 
     bool should_cow(size_t page_index) const;
     void set_should_cow(size_t page_index, bool);