CVE-2026-72290: KVM: s390: pci: Fix GISC refcount leak on AIF enable failure

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

KVM: s390: pci: Fix GISC refcount leak on AIF enable failure

kvms390giscregister() registers the guest ISC before pinning the guest interrupt forwarding pages and allocating the AISB bit. If any of the later setup steps fails, the function unwinds the pinned pages and other local state, but does not unregister the GISC reference. Add the missing kvms390giscunregister() to the error unwind path.