Kernel+SystemMonitor: Expose amount of per-process clean inode memory

This is memory that's loaded from an inode (file) but not modified in
memory, so still identical to what's on disk. This kind of memory can
be freed and reloaded transparently from disk if needed.

Libraries/LibCore/CProcessStatisticsReader.h

@@ -40,6 +40,7 @@ struct CProcessStatistics {
     size_t amount_resident;
     size_t amount_shared;
     size_t amount_dirty_private;
+    size_t amount_clean_inode;
     size_t amount_purgeable_volatile;
     size_t amount_purgeable_nonvolatile;
     int icon_id;