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

In the Linux kernel, the following vulnerability has been resolved:

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.