mm: bugfix #133: MapArea::copy_data does not need &mut PageTable

os/src/mm/memory_set.rs

@@ -63,7 +63,7 @@ impl MemorySet {
     fn push(&mut self, mut map_area: MapArea, data: Option<&[u8]>) {
         map_area.map(&mut self.page_table);
         if let Some(data) = data {
-            map_area.copy_data(&mut self.page_table, data);
+            map_area.copy_data(&self.page_table, data);
         }
         self.areas.push(map_area);
     }
@@ -341,7 +341,7 @@ impl MapArea {
     }
     /// data: start-aligned but maybe with shorter length
     /// assume that all frames were cleared before
-    pub fn copy_data(&mut self, page_table: &mut PageTable, data: &[u8]) {
+    pub fn copy_data(&mut self, page_table: &PageTable, data: &[u8]) {
         assert_eq!(self.map_type, MapType::Framed);
         let mut start: usize = 0;
         let mut current_vpn = self.vpn_range.get_start();