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

Currently, `CoherentAllocation` is concecptually a DMA coherent container
of a slice of `[T]` of runtime-checked length. Generalize it by creating
`dma::Coherent<T>` which can hold any value of `T`.
`Coherent::alloc_with_attrs` is implemented but not yet exposed, as I
believe we should not expose the way to obtain an uninitialized coherent
region.

`Coherent<[T]>` provides a `len` method instead of the previous `count()`
method to be consistent with methods on slices.

The existing type is re-defined as a type alias of `Coherent<[T]>` to ease
transition. Methods in use are not yet removed.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260320194626.36263-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/device.rs

@@ -575,7 +575,7 @@ pub struct CoreInternal;
 /// The bound context indicates that for the entire duration of the lifetime of a [`Device<Bound>`]
 /// reference, the [`Device`] is guaranteed to be bound to a driver.
 ///
-/// Some APIs, such as [`dma::CoherentAllocation`] or [`Devres`] rely on the [`Device`] to be bound,
+/// Some APIs, such as [`dma::Coherent`] or [`Devres`] rely on the [`Device`] to be bound,
 /// which can be proven with the [`Bound`] device context.
 ///
 /// Any abstraction that can guarantee a scope where the corresponding bus device is bound, should
@@ -584,7 +584,7 @@ pub struct CoreInternal;
 ///
 /// [`Devres`]: kernel::devres::Devres
 /// [`Devres::access`]: kernel::devres::Devres::access
-/// [`dma::CoherentAllocation`]: kernel::dma::CoherentAllocation
+/// [`dma::Coherent`]: kernel::dma::Coherent
 pub struct Bound;
 
 mod private {