Medium [CVE-2026-53116] use generic driver_override infrastructure

In the Linux kernel, the following vulnerability has been resolved: s390/ap: use generic driver_override infrastructure When the AP masks are updated via apmask_store() or aqmask_store(), ap_bus_revise_bindings() is called after ap_attr_mutex has been released.

This calls __ap_revise_reserved(), which accesses the driver_override field without holding any lock, racing against a concurrent driver_override_store() that may free the old string, resulting in a potential UAF. Fix this by using the driver-core driver_override infrastructure, which protects all accesses with an internal spinlock.

Note that unlike most other buses, the AP bus does not check driver_override in its match() callback; the override is checked in ap_device_probe() and __ap_revise_reserved() instead.

Also note that we do not enable the driver_override feature of struct bus_type, as AP - in contrast to most other buses - passes "" to sysfs_emit() when the driver_override pointer is NULL. Thus, printing "\n" instead of "(null)\n".

Additionally, AP has a custom counter that is modified in the corresponding custom driver_override_store(). A race condition occurs when AP masks are updated, leading to `__ap_revise_reserved()` accessing the `driver_override` field without proper locking.

This can result in a Use-After-Free (UAF) vulnerability, where memory is accessed after it has been freed.