linux-user: Fix type for SIOCATMARK ioctl

The SIOCATMARK ioctl takes an argument which should be a
pointer to an integer where the kernel will write the result.
We were incorrectly declaring it as TYPE_NULL which would mean
it would always fail (with EFAULT) when it should succeed.
Correct the type.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>