nvme-pci: rename the descriptor pools

They are used for both PRPs and SGLs, and we use descriptor elsewhere
when referring to their allocations, so use that name here as well.

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: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Leon Romanovsky <leon@kernel.org>