Add file RPC dup call

src/main.rs

@@ -64,6 +64,10 @@ impl file_rpc::Server for Serv {
     fn size(&self, fd: u64) -> Option<u64> {
         Some(self.open_files.get(fd as usize)?.2 as u64)
     }
+
+    fn dup(&self, fd: u64) -> Option<u64> {
+        Some(fd)
+    }
 }
 
 fn parse_mbr(mut disk: &File) -> Vec<MbrEntry> {