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.