nullb: fix error return code in null_init()

Fix to return error code -ENOMEM from the null_alloc_dev() error
handling case instead of 0, as done elsewhere in this function.

Fixes: 2984c868 ("nullb: factor disk parameters")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk.c

@@ -1991,8 +1991,10 @@ static int __init null_init(void)
 
 	for (i = 0; i < nr_devices; i++) {
 		dev = null_alloc_dev();
-		if (!dev)
+		if (!dev) {
+			ret = -ENOMEM;
 			goto err_dev;
+		}
 		ret = null_add_dev(dev);
 		if (ret) {
 			null_free_dev(dev);