rust: rbtree: remove unwrap in asserts

rust: rbtree: remove unwrap in asserts

Remove `unwrap` in asserts and replace it with `Option::Some`
matching. By doing it this way, the examples are more
descriptive, so it disambiguates the return type of
the `get(...)` and `next(...)`, because the `unwrap(...)`
can also be called on `Result`.

Signed-off-by: Daniel Sedlak <daniel@sedlak.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241123095033.41240-3-daniel@sedlak.dev
[ Reworded title slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>