nvdimm/bus: Fix potential use after free in asynchronous initialization

In the Linux kernel, the following vulnerability has been resolved:

nvdimm/bus: Fix potential use after free in asynchronous initialization

Dingisoul with KASAN reports a use after free if device_add() fails in

nd_async_device_register().

Commit b6eae0f61db2 ("libnvdimm: Hold reference on parent while

scheduling async init") correctly added a reference on the parent device

to be held until asynchronous initialization was complete. However, if

device_add() results in an allocation failure the ref count of the

device drops to 0 prior to the parent pointer being accessed. Thus

resulting in use after free.

The bug bot AI correctly identified the fix. Save a reference to the

parent pointer to be used to drop the parent reference regardless of the

outcome of device_add().