rust: alloc: implement `Borrow` and `BorrowMut` for `KBox`

rust: alloc: implement `Borrow` and `BorrowMut` for `KBox`

Implement `Borrow<T>` and `BorrowMut<T>` for `KBox<T>`. This allows
`KBox<T>` to be used in generic APIs asking for types implementing those
traits. `T` and `&mut T` also implement those traits allowing users to
use either owned, borrowed and heap-owned values.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250616-borrow_impls-v4-3-36f9beb3fe6a@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>