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>

rust/kernel/transmute.rs

@@ -57,6 +57,21 @@ pub unsafe trait AsBytes {
         // SAFETY: `data` is non-null and valid for reads of `len * sizeof::<u8>()` bytes.
         unsafe { core::slice::from_raw_parts(data, len) }
     }
+
+    /// Returns `self` as a mutable slice of bytes.
+    fn as_bytes_mut(&mut self) -> &mut [u8]
+    where
+        Self: FromBytes,
+    {
+        // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self`
+        // bi-directionally transmutable to `[u8; size_of_val(self)]`.
+        let data = core::ptr::from_mut(self).cast::<u8>();
+        let len = core::mem::size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes of `len * sizeof::<u8>()`
+        // bytes.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
 }
 
 macro_rules! impl_asbytes {