bugfix: 解决取消低地址映射时，错误的把重映射的物理页释放，从而导致的use after free问题。 (#321)

kernel/src/arch/x86_64/mm/mod.rs

@@ -610,10 +610,10 @@ impl LowAddressRemapping {
         assert!(mapper.as_mut().is_some());
         for i in 0..(Self::REMAP_SIZE / MMArch::PAGE_SIZE) {
             let vaddr = VirtAddr::new(i * MMArch::PAGE_SIZE);
-            let flusher = mapper
+            let (_, _, flusher) = mapper
                 .as_mut()
                 .unwrap()
-                .unmap(vaddr, true)
+                .unmap_phys(vaddr, true)
                 .expect("Failed to unmap frame");
             if flush == false {
                 flusher.ignore();