Unmap the old P4 page to create a basic guard page for the kernel stack

src/memory/paging/mod.rs

@@ -223,4 +223,8 @@ pub fn remap_the_kernel<A>(allocator: &mut A, boot_info: &BootInformation)
 
     let old_table = active_table.switch(new_table);
     println!("NEW TABLE!!!");
+
+    let old_p4_page = Page::containing_address(old_table.p4_frame.start_address());
+    active_table.unmap(old_p4_page, allocator);
+    println!("guard page at {:#x}", old_p4_page.start_address());
 }