nbd: Fix debugfs_create_dir error checking

The debugfs_create_dir function returns ERR_PTR in case of error, and the
only correct way to check if an error occurred is 'IS_ERR' inline function.
This patch will replace the null-comparison with IS_ERR.
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230512130533.98709-1-ivan.orlov0322@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -1666,7 +1666,7 @@ static int nbd_dev_dbg_init(struct nbd_device *nbd)
 		return -EIO;
 
 	dir = debugfs_create_dir(nbd_name(nbd), nbd_dbg_dir);
-	if (!dir) {
+	if (IS_ERR(dir)) {
 		dev_err(nbd_to_dev(nbd), "Failed to create debugfs dir for '%s'\n",
 			nbd_name(nbd));
 		return -EIO;
@@ -1692,7 +1692,7 @@ static int nbd_dbg_init(void)
 	struct dentry *dbg_dir;
 
 	dbg_dir = debugfs_create_dir("nbd", NULL);
-	if (!dbg_dir)
+	if (IS_ERR(dbg_dir))
 		return -EIO;
 
 	nbd_dbg_dir = dbg_dir;