linux-user: Detect fault in sched_rr_get_interval

Properly detect a fault when attempting to store into an invalid
struct timespec pointer.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
