nvme: make nvmf_register_transport require a create_ctrl callback

nvmf_create_ctrl() relys on the presence of a create_crtl callback in the
registered nvmf_transport_ops, so make nvmf_register_transport require one.

Update the available call-sites as well to reflect these changes.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/nvme/target/loop.c b/drivers/nvme/target/loop.c
index f3862e3..d1f06e7 100644
--- a/drivers/nvme/target/loop.c
+++ b/drivers/nvme/target/loop.c
@@ -724,8 +724,7 @@ static int __init nvme_loop_init_module(void)
 	ret = nvmet_register_transport(&nvme_loop_ops);
 	if (ret)
 		return ret;
-	nvmf_register_transport(&nvme_loop_transport);
-	return 0;
+	return nvmf_register_transport(&nvme_loop_transport);
 }
 
 static void __exit nvme_loop_cleanup_module(void)