book: ch4: MapArea::copy_data does not require &mut PageTable

Reference: rcore-os/rCore-Tutorial-v3#133

source/chapter4/5kernel-app-spaces.rst

@@ -107,7 +107,7 @@
         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);
         }
@@ -170,7 +170,7 @@
         }
         /// 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();