Use the core chrdev code to set up the link between the character device
and the nvme controller.  This allows us to get rid of the global list
of all controllers, and also ensures that we have both a reference to
the controller and the transport module before the open method of the
character device is called.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NSagi Grimberg <sgi@grimberg.me>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: NHannes Reinecke <hare@suse.com>