iio: core: Fix double free.

commit c1b03ab5 upstream.

When an error occurred during event registration memory was freed twice
resulting in kernel memory corruption and a crash in unrelated code.

The problem was caused by
	iio_device_unregister_eventset()
	iio_device_unregister_sysfs()

being called twice, once on the error path and then
again via iio_dev_release().

Fix this by making these two functions idempotent so they
may be called multiple times.

The problem was observed before applying
	78b33216 iio:core: Handle error when mask type is not separate
Signed-off-by: Martin Fuzzey <mfuzzey@parkeon.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
[bwh: Backported to 3.2:
 - Adjust filenames, context
 - Drop inapplicable change to iio_free_chan_devattr_list()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/staging/iio/industrialio-core.c

@@ -711,6 +711,7 @@ static void iio_device_unregister_sysfs(struct iio_dev *indio_dev)
 		iio_device_remove_and_free_read_attr(indio_dev, p);
 	}
 	kfree(indio_dev->chan_attr_group.attrs);
+	indio_dev->chan_attr_group.attrs = NULL;
 }
 
 static const char * const iio_ev_type_text[] = {
@@ -986,6 +987,7 @@ static int iio_device_register_eventset(struct iio_dev *indio_dev)
 error_free_setup_event_lines:
 	__iio_remove_event_config_attrs(indio_dev);
 	kfree(indio_dev->event_interface);
+	indio_dev->event_interface = NULL;
 error_ret:
 
 	return ret;