rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

Types that implement both `AsBytes` and `FromBytes` can be safely
modified as a slice of bytes. Add a `as_bytes_mut` method for that
purpose.

[acourbot@nvidia.com: use fully qualified `core::mem::size_of_val` to
build with Rust 1.78.]

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250801-as_bytes-v5-2-975f87d5dc85@nvidia.com


Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>