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

os/src/mm/memory_set.rs

@@ -330,7 +330,7 @@ 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();