nvme-pci: use a better encoding for small prp pool allocations

nvme-pci: use a better encoding for small prp pool allocations

There is plenty of unused space in the iod next to nr_descriptors.
Add a separate bool (which could be condensed to a single bit once we
start running out of space) to encode that the transfer is using
the full page sized pool, and use a normal 0..n count for the number of
descriptors.

Signed-off-by: Christoph Hellwig <hch@lst.de>