rust: str: replace unwraps with question mark operators

rust: str: replace unwraps with question mark operators

Simplify the error handling by replacing unwraps with the question
mark operator. Furthermore, unwraps can convey a wrong impression that
unwrapping is fine in general, thus this patch removes this unwrapping.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/CANiq72nsK1D4NuQ1U7NqMWoYjXkqQSj4QuUEL98OmFbq022Z9A@mail.gmail.com/
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Daniel Sedlak <daniel@sedlak.dev>
Link: https://lore.kernel.org/r/20241123095033.41240-5-daniel@sedlak.dev
[ Slightly reworded commit. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>