dma-buf: system_heap: No separate allocation for attachment sg_tables

dma-buf: system_heap: No separate allocation for attachment sg_tables

struct dma_heap_attachment is a separate allocation from the struct
sg_table it contains, but there is no reason for this. Let's use the
slab allocator just once instead of twice for dma_heap_attachment.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://lore.kernel.org/r/20250417180943.1559755-1-tjmercier@google.com