dma-mapping: Add internal shared allocation attribute

DMA_ATTR_CC_SHARED describes an existing DMA mapping whose backing memory
is already shared, or decrypted, for confidential computing. It is a
mapping attribute: callers use it to request a shared DMA address encoding
for memory that has already been prepared for shared DMA.

Allocation paths need a related but different state. Once the DMA core
decides that an allocation must use shared backing pages, the lower-level
allocation helpers need to select shared pools, decrypt newly allocated
pages, derive the DMA address with the shared-memory translation and
restore encryption on free. That state is internal to the DMA-mapping
implementation and should not be passed by drivers to dma_alloc_attrs().

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20260717180442.110954-10-aneesh.kumar@kernel.org
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

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