Kernel: rename MMU::m_page_directory_pointer_table -> m_highest_paging_stuct

This might not always be pdpt. In x86_64 it is pml4/pml5

kernel/include/kernel/MMU.h

@@ -19,5 +19,5 @@ private:
 	MMU();
 
 private:
-	uint64_t* m_page_descriptor_pointer_table;
+	uint64_t* m_highest_paging_struct;
 };