iio: accel: fxls8962af: Fix use after free in fxls8962af_fifo_flush

iio: accel: fxls8962af: Fix use after free in fxls8962af_fifo_flush

fxls8962af_fifo_flush() uses indio_dev->active_scan_mask (with
iio_for_each_active_channel()) without making sure the indio_dev
stays in buffer mode.
There is a race if indio_dev exits buffer mode in the middle of the
interrupt that flushes the fifo. Fix this by calling
synchronize_irq() to ensure that no interrupt is currently running when
disabling buffer mode.

Unable to handle kernel NULL pointer dereference at virtual address 00000000 when read
[...]
_find_first_bit_le from fxls8962af_fifo_flush+0x17c/0x290
fxls8962af_fifo_flush from fxls8962af_interrupt+0x80/0x178
fxls8962af_interrupt from irq_thread_fn+0x1c/0x7c
irq_thread_fn from irq_thread+0x110/0x1f4
irq_thread from kthread+0xe0/0xfc
kthread from ret_from_fork+0x14/0x2c

Fixes: 79e3a5bdd9ef ("iio: accel: fxls8962af: add hw buffered sampling")
Cc: stable@vger.kernel.org
Suggested-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://patch.msgid.link/20250603-fxlsrace-v2-1-5381b36ba1db@geanix.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>