kernel: fix kfdwrite signature to match RB-modified trait (add arg param, cloexec field)

The RB fork's KernelScheme trait has an extra 'arg: u64' parameter in
kfdwrite (not in upstream). Also RB's FileDescriptor has a 'cloexec' field.
Both are now handled correctly.

src/scheme/proc.rs

@@ -940,6 +940,7 @@ impl KernelScheme for ProcScheme {
         id: usize,
         descs: Vec<Arc<context::file::LockedFileDescription>>,
         flags: CallFlags,
+        _arg: u64,
         metadata: &[u64],
         token: &mut CleanLockToken,
     ) -> Result<usize> {
@@ -969,7 +970,7 @@ impl KernelScheme for ProcScheme {
             file.close(token)?;
         }
 
-        let mut file = descs.get(0).unwrap();
+        let file = descs.get(0).unwrap();
         let mut context = context.write(token.token());
         let (context, mut token) = context.token_split();
         context
@@ -977,6 +978,7 @@ impl KernelScheme for ProcScheme {
                 FileHandle(usize::try_from(*target_fd).map_err(|_| Error::new(EBADFD))?),
                 context::file::FileDescriptor {
                     description: file.clone(),
+                    cloexec: false,
                 },
                 &mut token,
             )