rust: file: use to_result for error handling

Simplifies error handling by replacing the manual check
of the return value with the `to_result` helper.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Link: https://lore.kernel.org/20250821091001.28563-1-work@onurozkan.dev


Signed-off-by: Christian Brauner <brauner@kernel.org>