完善了 sys_munmap 系统调用

要提交的变更：
      修改：     src/mm/memory_set.rs
      修改：     src/mm/mod.rs
      修改：     src/mm/page_table.rs
      修改：     src/syscall/process.rs
      修改：     src/task/mod.rs
      修改：     src/task/processor.rs

os/src/mm/memory_set.rs

@@ -284,6 +284,23 @@ impl MemorySet {
             -1
         }
     }
+
+    /// 取消当前进程指定区间的映射
+    /// 样例比较简单所以直接以开头页面进行删除了
+    pub fn unmap_vpnrange(&mut self, start: VirtPageNum, end: VirtPageNum) -> isize {
+        if let Some(_) = self.areas
+            .iter()
+            .position(|area| {
+                let (l, r) = (area.vpn_range.get_start(), area.vpn_range.get_end());
+                l == start && r == end
+            }) 
+        {
+            self.remove_area_with_start_vpn(start);
+            0
+        } else {
+            -1
+        }
+    }
 }
 /// map area structure, controls a contiguous piece of virtual memory
 pub struct MapArea {

os/src/mm/page_table.rs

@@ -158,8 +158,8 @@ impl PageTable {
         return None;
     }
     pub fn is_map(&self, vpn: VirtPageNum) -> bool {
-        if let Some(_) = self.find_pte(vpn) {
-            true
+        if let Some(pte) = self.find_pte(vpn) {
+            pte.is_valid()
         } else {
             false
         }

os/src/task/mod.rs

@@ -247,12 +247,12 @@ impl TaskManager {
         curr_task.memory_set.insert_framed_area(start_va, end_va, permission);
     }
 
-    /// unmap [start_va, end_va]
-    pub fn curr_munmap_with_start_vpn(&self, start_vpn: VirtPageNum) -> isize {
+    /// unmap [start_va, end _va]
+    pub fn curr_munmap(&self, start_vpn: VirtPageNum, end_vpn: VirtPageNum) -> isize {
         let mut inner = self.inner.exclusive_access();
         let curr_id = inner.current_task;
         let curr_task = &mut inner.tasks[curr_id];
-        curr_task.memory_set.remove_area_with_start_vpn(start_vpn)
+        curr_task.memory_set.unmap_vpnrange(start_vpn, end_vpn) 
     }
 }
 
@@ -346,9 +346,10 @@ pub fn curr_mmap(start: usize, len: usize, mut port: usize) -> isize {
 pub fn curr_munmap(start: usize, len: usize) -> isize {
     let start_va = VirtAddr::from(start);
     let end_va = VirtAddr::from(start + len);
-    if !start_va.aligned() || curr_vpnrange_exist_unmap(start_va.floor(), end_va.ceil()) {
+    if !start_va.aligned() ||
+    curr_vpnrange_exist_unmap(start_va.floor(), end_va.ceil()) {
         -1
     } else {
-        TASK_MANAGER.curr_munmap_with_start_vpn(start_va.floor())
+        TASK_MANAGER.curr_munmap(start_va.floor(), end_va.ceil())
     }
 }