create single-segment HMBs when using IOMMU

[Christoph Hellwig]

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.

Diffstat:
 pci.c |   74 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 62 insertions(+), 12 deletions(-)