feat: vspace: entry: add clear api

kernel/src/arch/riscv/vspace/entry.rs

@@ -117,6 +117,10 @@ impl EntryOps for Entry {
         self.0 = (self.0 & !PTEFlags::all().bits()) | flags.bits();
     }
 
+    fn clear(&mut self) {
+        self.0 = 0;
+    }
+
     fn is_valid(&self) -> bool {
         self.0 & PTEFlags::VALID.bits() != 0
     }

kernel/src/vspace/entry.rs

@@ -23,6 +23,7 @@ pub trait EntryOps: Clone + Copy + Debug {
 
     fn set_paddr(&mut self, addr: PhysAddr);
     fn set_attr(&mut self, attr: MapAttr);
+    fn clear(&mut self);
 
     fn is_valid(&self) -> bool;
     fn is_leaf(&self) -> bool;