ublk_drv: fix an IS_ERR() vs NULL check

The blk_mq_alloc_disk_for_queue() doesn't return error pointers, it
returns NULL on error.

Fixes: cebbe577 ("ublk_drv: fix request queue leak")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/YtVAgedTsQVK1oTM@kili


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