nvme: cleanup the nvme_init_integrity calling conventions

Handle the no metadata support case in nvme_init_integrity as well to
simplify the calling convention and prepare for future changes in the
area.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>