rust: str: move SAFETY comment in front of unsafe block

SAFETY comments should immediately precede the unsafe block they
justify. Move assignment to `bar` past comment as it is safe.

Signed-off-by: Valentin Obst <kernel@valentinobst.de>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240131-doc-fixes-v3-v3-6-0c8af94ed7de@valentinobst.de


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>