iio: adc: at91-sama5d2_adc: Fix potential use-after-free in sama5d2_adc driver

iio: adc: at91-sama5d2_adc: Fix potential use-after-free in sama5d2_adc driver

at91_adc_interrupt can call at91_adc_touch_data_handler function
to start the work by schedule_work(&st->touch_st.workq).

If we remove the module which will call at91_adc_remove to
make cleanup, it will free indio_dev through iio_device_unregister but
quite a bit later. While the work mentioned above will be used. The
sequence of operations that may lead to a UAF bug is as follows:

CPU0                                      CPU1

                                     | at91_adc_workq_handler
at91_adc_remove                      |
iio_device_unregister(indio_dev)     |
//free indio_dev a bit later         |
                                     | iio_push_to_buffers(indio_dev)
                                     | //use indio_dev

Fix it by ensuring that the work is canceled before proceeding with
the cleanup in at91_adc_remove.

Fixes: 23ec2774f1cc ("iio: adc: at91-sama5d2_adc: add support for position and pressure channels")
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>