KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler

In the Linux kernel, the following vulnerability has been resolved:

KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler

We do check for target CPU == -1, but this might change at the time we are going to use it. Hold the physical target CPU in a local variable to avoid out-of-bound accesses to the cpu arrays.