nvme: remove unused parameter

nvme_alloc_ns_head() doesn't use the 'struct nvme_id_ns' parameter.
Remove it, and update caller accordingly.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>