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

Add a separate flag to encode that the transfer is using the small
page sized pool, and use a normal 0..n count for the number of
descriptors.

Contains improvements and suggestions from Kanchan Joshi
<joshi.k@samsung.com> and Leon Romanovsky <leon@kernel.org>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Leon Romanovsky <leon@kernel.org>