Add file RPC dup call

src/main.rs

@@ -58,6 +58,10 @@ impl file_rpc::Server for Serv {
         let (file, _mount_id) = &files[fd as usize];
         Some(file.metadata().len())
     }
+
+    fn dup(&self, fd: u64) -> Option<u64> {
+        Some(fd)
+    }
 }
 
 fn main() {