nvme-pci: use mapped entries for sgl decision

The driver uses the dma entries for setting up its command's SGL/PRP
lists. The dma mapping might have fewer entries than the physical
segments, so check the dma mapped count to determine which nvme data
layout method is more optimal.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>