The GPIO_GET_LINEEVENT_IOCTL currently ignores unknown or undefined
linehandle and lineevent flags. From a backwards and forwards compatibility
viewpoint it is highly desirable to reject unknown flags though.

On one hand an application that is using newer flags and is running on
an older kernel has no way to detect if the new flags were handled
correctly if they are silently discarded.

On the other hand an application that (accidentally) passes undefined flags
will run fine on an older kernel, but may break on a newer kernel when
these flags get defined.

Ensure that requests that have undefined flags set are rejected with an
error, rather than silently discarding the undefined flags.

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