samples: rust: debugfs: use core::ffi::CStr method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoiding methods that only exist on the latter.

This backslid in commit d4a5d397c7 ("samples: rust: Add scoped debugfs
sample driver").

Link: https://patch.msgid.link/20251019213049.2060970-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

samples/rust/rust_debugfs_scoped.rs

@@ -38,7 +38,7 @@ fn remove_file_write(
     mod_data
         .devices
         .lock()
-        .retain(|device| device.name.as_bytes() != to_remove.as_bytes());
+        .retain(|device| device.name.to_bytes() != to_remove.to_bytes());
     Ok(())
 }