hw/nvme: save reftag when generating pi

Prepare nvme_dif_pract_generate_dif() and nvme_dif_check() to be
callable in smaller increments by making the reftag a pointer parameter
updated by the function.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
3 files changed