rust: add safety comment in workqueue traits

Add missing safety comments for the implementation of the unsafe traits
WorkItemPointer and RawWorkItem for Arc<T> in workqueue.rs

Link: https://github.com/Rust-for-Linux/linux/issues/351

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Co-developed-by: Vangelis Mamalakis <mamalakis@google.com>
Signed-off-by: Vangelis Mamalakis <mamalakis@google.com>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Konstantin Andrikopoulos <kernel@mandragore.io>
Signed-off-by: Tejun Heo <tj@kernel.org>