create single-segment HMBs when using IOMMU

[Keith Busch]

On Fri, Nov 01, 2024 at 05:40:03AM +0100, Christoph Hellwig wrote:
> Hi all,
> 
> NVMe controllers usually have a limit of how many HMB descriptors they
> support, and even if they support multiple they usually prefer less of
> them.  When running with an IOMMU, the DMA API can coalesce virtually
> discontiguous segments, and using the right API doesn't even eat up
> vmalloc space for that.

This all looks good to me. Thanks, applied to nvme-6.13.