rust: add dma coherent allocator abstraction

rust: add dma coherent allocator abstraction

Add a simple dma coherent allocator rust abstraction. Based on
Andreas Hindborg's dma abstractions from the rnvme driver, which
was also based on earlier work by Wedson Almeida Filho.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250317185345.2608976-3-abdiel.janulgue@gmail.com
Nacked-by: Christoph Hellwig <hch@lst.de>
[ Removed period. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>