nvme: simplify block device ioctl handling for the !multipath case

Only use the existing ioctl handler for the multipath case, and add a
simpler one that reverts to the pre-multipath case for not shared
use case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Javier González <javier.gonz@samsung.com>