Let PageDirectory have a PhysicalPage for the PDB instead of being the PDB.

This is more efficient than using the wasteful kmalloc_aligned() approach
to allocation. It also allows much tighter encapsulation of the class.

Kernel/Process.h

@@ -238,7 +238,7 @@ private:
 
     int alloc_fd();
 
-    PageDirectory* m_page_directory { nullptr };
+    OwnPtr<PageDirectory> m_page_directory;
 
     Process* m_prev { nullptr };
     Process* m_next { nullptr };