rust: dma: implement BinaryWriter for Coherent<[u8]>

rust: dma: implement BinaryWriter for Coherent<[u8]>

Implement the BinaryWriter trait for Coherent<[u8]>, enabling DMA
coherent allocations to be exposed as readable binary files.  The
implementation handles offset tracking and bounds checking, copying data
from the coherent allocation to userspace via write_dma().

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Tested-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260319212658.2541610-4-ttabi@nvidia.com
[ Rebase onto Coherent<T> changes. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>