mm/hmm: Indicate that HMM requires DMA coherency

HMM mirroring can work on coherent systems without SWIOTLB path only.
Until introduction of DMA_ATTR_REQUIRE_COHERENT, there was no reliable
way to indicate that and various approximation was done:

int hmm_dma_map_alloc(struct device *dev, struct hmm_dma_map *map,
                      size_t nr_entries, size_t dma_entry_size)
{
<...>
        /*
         * The HMM API violates our normal DMA buffer ownership rules and can't
         * transfer buffer ownership.  The dma_addressing_limited() check is a
         * best approximation to ensure no swiotlb buffering happens.
         */
        dma_need_sync = !dev->dma_skip_sync;
        if (dma_need_sync || dma_addressing_limited(dev))
                return -EOPNOTSUPP;

So let's mark mapped buffers with DMA_ATTR_REQUIRE_COHERENT attribute
to prevent DMA debugging warnings for cache overlapped entries.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>