gpio: GPIO_GET_LINEEVENT_IOCTL: Validate line offset

The line offset that is used as an index into the descs array is provided
by userspace and might go beyond the bounds of the array. If that happens
undefined behavior will occur.

Make sure that the offset is within the bounds of the desc array and reject
any requests that specify a value outside of it.

Cc: stable@vger.kernel.org
Fixes: 61f922db ("gpio: userspace ABI for reading GPIO line events")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>