iio: adc: ina2xx: avoid kthread_stop() with stale task_struct

iio: adc: ina2xx: avoid kthread_stop() with stale task_struct

When the buffer is enabled for ina2xx driver, a dedicated kthread is
invoked to capture mesurement data.  When the buffer is disabled, the
kthread is stopped.

However if the kthread gets register access errors, it immediately exits
and when the malfunctional buffer is disabled, the stale task_struct
pointer is accessed as there is no kthread to be stopped.

A similar issue in the usbip driver is prevented by kthread_get_run and
kthread_stop_put helpers by increasing usage count of the task_struct.
This change applies the same solution.

Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Fixes: c43a102e67db ("iio: ina2xx: add support for TI INA2xx Power Monitors")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>