rbd: set the 'device' link in sysfs

The rbd driver already provides additional information in sysfs
under /sys/bus/rbd, so we should set the 'device' link in the block
device to reference this information.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>