Correctly impl /dev/null

kernel/src/fs/devnull.rs

@@ -33,10 +33,7 @@ macro_rules! impl_inode {
 
 impl INode for DevNull {
     fn read_at(&self, offset: usize, buf: &mut [u8]) -> Result<usize> {
-        for i in buf.iter_mut() {
-            *i = 0
-        }
-        Ok(buf.len())
+        Ok(0)
     }
     fn write_at(&self, _offset: usize, buf: &[u8]) -> Result<usize> {
         Ok(buf.len())