ublk: fold __ublk_create_dev into ublk_ctrl_add_dev

Fold __ublk_create_dev into its only caller to avoid the packing and
unpacking of the return value into an ERR_PTR.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-7-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>