refactor(rm/linux): remove mut from dir_fd parameter in safe_remove_dir_recursive_impl

The `dir_fd` parameter was marked as mutable but is not modified within the function,
so removing `mut` enforces Rust's immutability-by-default principle and improves code clarity.

Author: mattsu2020

src/uu/rm/src/platform/linux.rs

@@ -312,7 +312,7 @@ pub fn safe_remove_dir_recursive(
 
 pub fn safe_remove_dir_recursive_impl(
     path: &Path,
-    mut dir_fd: DirFd,
+    dir_fd: DirFd,
     options: &Options,
     progress_bar: Option<&ProgressBar>,
 ) -> bool {