Kernel: Use an IntrusiveRedBlackTree for storing the cr3 mappings

This ensures we don't allocate when intializing the PageDirectory.

Kernel/Memory/PageDirectory.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/HashMap.h>
+#include <AK/IntrusiveRedBlackTree.h>
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <Kernel/Forward.h>
@@ -46,6 +47,9 @@ public:
 
     RecursiveSpinlock& get_lock() { return m_lock; }
 
+    // This has to be public to let the global singleton access the member pointer
+    IntrusiveRedBlackTreeNode<FlatPtr, PageDirectory, RawPtr<PageDirectory>> m_tree_node;
+
 private:
     PageDirectory();