ser_gigaset: use container_of() instead of detour

The purpose of gigaset_device_release() is to kfree() the struct
ser_cardstate that contains our struct device. This is done via a bit of
a detour. First we make our struct device's driver_data point to the
container of our struct ser_cardstate (which is a struct cardstate). In
gigaset_device_release() we then retrieve that driver_data again. And
after that we finally kfree() the struct ser_cardstate that was saved in
the struct cardstate.

All of this can be achieved much easier by using container_of() to get
from our struct device to its container, struct ser_cardstate. Do so.

Note that at the time the detour was implemented commit b8b2c7d
("base/platform: assert that dev_pm_domain callbacks are called
unconditionally") had just entered the tree. That commit disconnected
our platform_device and our platform_driver. These were reconnected
again in v4.5-rc2 through commit 25cad69 ("base/platform: Fix
platform drivers with no probe callback"). And one of the consequences
of that fix was that it broke the detour via driver_data. That's because
it made __device_release_driver() stop being a NOP for our struct device
and actually do stuff again. One of the things it now does, is setting
our driver_data to NULL. That, in turn, makes it impossible for
gigaset_device_release() to get to our struct cardstate. Which has the
net effect of leaking a struct ser_cardstate at every call of this
driver's tty close() operation. So using container_of() has the
additional benefit of actually working.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/gigaset/ser-gigaset.c

@@ -373,13 +373,7 @@ static void gigaset_freecshw(struct cardstate *cs)
 
 static void gigaset_device_release(struct device *dev)
 {
-	struct cardstate *cs = dev_get_drvdata(dev);
-
-	if (!cs)
-		return;
-	dev_set_drvdata(dev, NULL);
-	kfree(cs->hw.ser);
-	cs->hw.ser = NULL;
+	kfree(container_of(dev, struct ser_cardstate, dev.dev));
 }
 
 /*
@@ -408,7 +402,6 @@ static int gigaset_initcshw(struct cardstate *cs)
 		cs->hw.ser = NULL;
 		return rc;
 	}
-	dev_set_drvdata(&cs->hw.ser->dev.dev, cs);
 
 	tasklet_init(&cs->write_tasklet,
 		     gigaset_modem_fill, (unsigned long) cs);