libnvdimm: Hold reference on parent while scheduling async init

BugLink: https://bugs.launchpad.net/bugs/1810947

commit b6eae0f6 upstream.

Unlike asynchronous initialization in the core we have not yet associated
the device with the parent, and as such the device doesn't hold a reference
to the parent.

In order to resolve that we should be holding a reference on the parent
until the asynchronous initialization has completed.

Cc: <stable@vger.kernel.org>
Fixes: 4d88a97a ("libnvdimm: ...base ... infrastructure")
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/nvdimm/bus.c

@@ -158,6 +158,8 @@ static void nd_async_device_register(void *d, async_cookie_t cookie)
 		put_device(dev);
 	}
 	put_device(dev);
+	if (dev->parent)
+		put_device(dev->parent);
 }
 
 static void nd_async_device_unregister(void *d, async_cookie_t cookie)
@@ -175,6 +177,8 @@ static void nd_async_device_unregister(void *d, async_cookie_t cookie)
 void __nd_device_register(struct device *dev)
 {
 	dev->bus = &nvdimm_bus_type;
+	if (dev->parent)
+		get_device(dev->parent);
 	get_device(dev);
 	async_schedule_domain(nd_async_device_register, dev,
 			&nd_async_domain);