nvme-pci: use dma_alloc_noncontigous if possible

Use dma_alloc_noncontigous to allocate a single IOVA-contigous segment
when backed by an IOMMU.  This allow to easily use bigger segments and
avoids running into segment limits if we can avoid it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>