kvm: s390: Reject memory region operations for ucontrol VMs

In the Linux kernel, the following vulnerability has been resolved: kvm: s390: Reject memory region operations for ucontrol VMs This change rejects the KVM_SET_USER_MEMORY_REGION and KVM_SET_USER_MEMORY_REGION2 ioctls when called on a ucontrol VM. This is necessary since ucontrol VMs have kvm->arch.gmap set to 0 and would thus result in a null pointer dereference further in. Memory management needs to be performed in userspace and using the ioctls KVM_S390_UCAS_MAP and KVM_S390_UCAS_UNMAP. Also improve s390 specific documentation for KVM_SET_USER_MEMORY_REGION and KVM_SET_USER_MEMORY_REGION2. [frankja@linux.ibm.com: commit message spelling fix, subject prefix fix]