s390/dasd: fix list corruption of pavgroup group list

dasd_alias_add_device() moves devices to the active_devices list in case
of a scheduled LCU update regardless if they have previously been in a
pavgroup or not.

Example: device A and B are in the same pavgroup.

Device A has already been in a pavgroup and the private->pavgroup pointer
is set and points to a valid pavgroup. While going through dasd_add_device
it is moved from the pavgroup to the active_devices list.

In parallel device B might be removed from the same pavgroup in
remove_device_from_lcu() which in turn checks if the group is empty
and deletes it accordingly because device A has already been removed from
there.

When now device A enters remove_device_from_lcu() it is tried to remove it
from the pavgroup again because the pavgroup pointer is still set and again
the empty group will be cleaned up which leads to a list corruption.

Fix by setting private->pavgroup to NULL in dasd_add_device.

If the device has been the last device on the pavgroup an empty pavgroup
remains but this will be cleaned up by the scheduled lcu_update which
iterates over all existing pavgroups.

Fixes: 8e09f215 ("[S390] dasd: add hyper PAV support to DASD device driver, part 1")
Cc: stable@vger.kernel.org
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/s390/block/dasd_alias.c

@@ -642,6 +642,7 @@ int dasd_alias_add_device(struct dasd_device *device)
 	}
 	if (lcu->flags & UPDATE_PENDING) {
 		list_move(&device->alias_list, &lcu->active_devices);
+		private->pavgroup = NULL;
 		_schedule_lcu_update(lcu, device);
 	}
 	spin_unlock_irqrestore(&lcu->lock, flags);