nvme: make SG_IO support optional

Translation SCSI commands to NVMe commands is rather pointless in general
as applications must not expext to be able to use SCSI commands on a
generic block device.

Make the huge translation layer optional and hope no one will ever enable
it in the future.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>