Claude review: rust: dma: add generalized container for types other than slices

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This is the core refactoring patch. Several observations:

**Typo in commit message**: "concecptually" should be "conceptually".

**Vague invariant wording**: The updated invariant says:
```
/// - The size in bytes of the allocation is equal to size information via pointer.
```
This is much less precise than the original. Consider something like: "The size in bytes of the allocation is equal to `T::size(cpu_addr.as_ptr())`."

**`as_mut_ptr` returns `*mut T` from `&self`**: This is intentional for DMA memory (the `#[expect(clippy::mut_from_ref)]` on `as_mut()` confirms this), but the `as_mut_ptr` method itself lacks any documentation about why `&self` suffices. A brief note on the safety rationale (same as `as_mut`) would be helpful.

**Unnecessary `?` operator**:
```rust
if len == 0 {
    Err(EINVAL)?;
}
```
`return Err(EINVAL)` is more direct and idiomatic.

**Dependency on `KnownSize`**: `crate::ptr::KnownSize` is imported but doesn't exist in the current tree. This is a hard dependency on unmerged work that should be noted in the cover letter.

**`alloc_slice_with_attrs` overflow error code**: The original used `EOVERFLOW` for the size multiplication overflow, but the new code uses `ENOMEM`:
```rust
let size = core::mem::size_of::<T>().checked_mul(len).ok_or(ENOMEM)?;
```
`EOVERFLOW` was arguably more descriptive for an arithmetic overflow; `ENOMEM` is technically inaccurate here (it's not that memory is exhausted, it's that the requested size is invalid).

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