rust: kernel: add blank lines in front of code blocks

Throughout the code base, blank lines are used before starting a code
block. Adapt outliers to improve consistency within the kernel crate.

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-v3-9-0c8af94ed7de@valentinobst.de


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