block/rnbd-clt: Move add_disk(dev->gd) to rnbd_clt_setup_gen_disk

It makes more sense to add gendisk in rnbd_clt_setup_gen_disk, instead
of do it in rnbd_clt_map_device.

Signed-off-by: Guoqing Jiang <guoqing.jiang@gmx.com>
Reviewed-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210419073722.15351-6-gi-oh.kim@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>