net/rose: prevent integer overflows in rose_setsockopt()

In the Linux kernel, the following vulnerability has been resolved: net/rose: prevent integer overflows in rose_setsockopt() In case of possible unpredictably large arguments passed to rose_setsockopt() and multiplied by extra values on top of that, integer overflows may occur. Do the safest minimum and fix these issues by checking the contents of 'opt' and returning -EINVAL if they are too large. Also, switch to unsigned int and remove useless check for negative 'opt' in ROSE_IDLE case.