s390/vfio-ap: fix memory leak in vfio_ap device driver

In the Linux kernel, the following vulnerability has been resolved: s390/vfio-ap: fix memory leak in vfio_ap device driver The device release callback function invoked to release the matrix device uses the dev_get_drvdata(device *dev) function to retrieve the pointer to the vfio_matrix_dev object in order to free its storage. The problem is, this object is not stored as drvdata with the device; since the kfree function will accept a NULL pointer, the memory for the vfio_matrix_dev object is never freed. Since the device being released is contained within the vfio_matrix_dev object, the container_of macro will be used to retrieve its pointer.