CVE-2026-53116: s390/ap: use generic driver_override infrastructure

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

s390/ap: use generic driveroverride infrastructure

When the AP masks are updated via apmaskstore() or aqmaskstore(), apbusrevisebindings() is called after apattrmutex has been released.

This calls aprevisereserved(), which accesses the driveroverride field without holding any lock, racing against a concurrent driveroverridestore() that may free the old string, resulting in a potential UAF.

Fix this by using the driver-core driveroverride infrastructure, which protects all accesses with an internal spinlock.

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

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

Additionally, AP has a custom counter that is modified in the corresponding custom driveroverridestore().