Kernel+SystemMonitor: Expose the number of set CoW bits in each Region

This number tells us how many more pages in a given region will trigger
a CoW fault if written to.

Applications/SystemMonitor/ProcessMemoryMapWidget.cpp

@@ -37,6 +37,7 @@ ProcessMemoryMapWidget::ProcessMemoryMapWidget(GWidget* parent)
             return "Volatile";
         return "Non-volatile";
     });
+    pid_vm_fields.empend("cow_pages", "# CoW", TextAlignment::CenterRight);
     pid_vm_fields.empend("name", "Name", TextAlignment::CenterLeft);
     m_json_model = GJsonArrayModel::create({}, move(pid_vm_fields));
     m_table_view->set_model(GSortingProxyModel::create(*m_json_model));

Kernel/FileSystem/ProcFS.cpp

@@ -273,6 +273,7 @@ Optional<KBuffer> procfs$pid_vm(InodeIdentifier identifier)
         region_object.add("address", region.vaddr().get());
         region_object.add("size", (u32)region.size());
         region_object.add("amount_resident", (u32)region.amount_resident());
+        region_object.add("cow_pages", region.cow_pages());
         region_object.add("name", region.name());
     }
     array.finish();

Kernel/VM/Region.cpp

@@ -113,6 +113,16 @@ int Region::commit()
     return 0;
 }
 
+u32 Region::cow_pages() const
+{
+    if (!m_cow_map)
+        return 0;
+    u32 count = 0;
+    for (int i = 0; i < m_cow_map->size(); ++i)
+        count += m_cow_map->get(i);
+    return count;
+}
+
 size_t Region::amount_resident() const
 {
     size_t bytes = 0;

Kernel/VM/Region.h

@@ -105,6 +105,8 @@ public:
     bool should_cow(size_t page_index) const;
     void set_should_cow(size_t page_index, bool);
 
+    u32 cow_pages() const;
+
     void set_writable(bool b)
     {
         if (b)