NVMe: Fix error handling of class_create("nvme")

class_create() returns ERR_PTR on failure,
so IS_ERR() should be used instead of check for NULL.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nvme-core.c

@@ -3163,8 +3163,10 @@ static int __init nvme_init(void)
 		nvme_char_major = result;
 
 	nvme_class = class_create(THIS_MODULE, "nvme");
-	if (!nvme_class)
+	if (IS_ERR(nvme_class)) {
+		result = PTR_ERR(nvme_class);
 		goto unregister_chrdev;
+	}
 
 	result = pci_register_driver(&nvme_driver);
 	if (result)