rust: block: simplify Result<()> in validate_block_size return

`Result` is used in place of `Result<()>` because the default type
parameters are unit `()` and `Error` types, which are automatically
inferred. Thus keep the usage consistent throughout codebase.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1128


Signed-off-by: Manas <manas18244@iiitd.ac.in>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20241118-simplify-result-v3-1-6b1566a77eab@iiitd.ac.in


Signed-off-by: Jens Axboe <axboe@kernel.dk>