Merge branch 'ap-driver-override' into features

Harald Freudenberger says:

====================

Support for driver override on AP queues.

Add a new sysfs attribute driver_override the AP queue's directory. Writing
in a string overrides the default driver determination and the drivers are
matched against this string instead. This overrules the driver binding
determined by the apmask/aqmask bitmask fields. With the write to the
attribute a check is done if the queue is in use by an mdev device.
If this is true, the write is aborted and EBUSY is returned.

As there exists some tooling for this kind of driver_override
(see package driverctl) the AP bus behavior for re-binding
should be compatible to this. The steps for a driver_override are:
 1) unbind the current driver from the device. For example
    echo "17.0005" > /sys/devices/ap/card17/17.0005/driver/unbind
 2) set the new driver for this device in the sysfs
    driver_override attribute. For example
    echo "vfio_ap" > /sys//devices/ap/card17/17.0005/driver_override
 3) trigger a bus reprobe of this device. For example
    echo "17.0005" > /sys/bus/ap/drivers_probe
With the driverctl package this is more comfortable and
the settings get persisted:
  driverctl -b ap set-override 17.0005 vfio_ap
and unset with
  driverctl -b ap unset-override 17.0005

====================

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

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