Add a PageDirectory::flush() that does nothing if another PD is active.

This way callers can just flush() every time after making any modification
and the PageDirectory itself will decide if TLB invalidation is necessary.

Kernel/MemoryManager.h

@@ -61,7 +61,7 @@ public:
     dword cr3() const { return m_directory_page->paddr().get(); }
     dword* entries() { return reinterpret_cast<dword*>(cr3()); }
 
-    bool is_active() const;
+    void flush(LinearAddress);
 
 private:
     RetainPtr<PhysicalPage> m_directory_page;