rust: list: replace unwrap() with ? in doctest examples

Using `unwrap()` in kernel doctests can cause panics on error and may
give newcomers the mistaken impression that panicking is acceptable
in kernel code.

Replace all `.unwrap()` calls in `kernel::list`
examples with `.ok_or(EINVAL)?` so that errors are properly propagated.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1164


Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250527204928.5117-1-albinbabuvarghese20@gmail.com


[ Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>