Fix "clone grant using fmap"

This does the same as the previous MR, but fixes the issue where the
parent process got the mapping (and at the wrong address) and not the
child process.

src/context/file.rs

@@ -7,7 +7,7 @@ use crate::scheme::{self, SchemeNamespace, SchemeId};
 use crate::syscall::error::{Result, Error, EBADF};
 
 /// A file description
-#[derive(Debug)]
+#[derive(Clone, Copy, Debug)]
 pub struct FileDescription {
     /// The namespace the file was opened from (used for debugging)
     pub namespace: SchemeNamespace,