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.