KVM: ia64: fix the error code of ioctl KVM_IA64_VCPU_GET_STACK failure

The ioctl KVM_IA64_VCPU_GET_STACK does not set the error code if
copy_to_user() fail, and 0 will be return, we should use -EFAULT
instead of 0 in this case, so this patch fixed it.
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/ia64/kvm/kvm-ia64.c

@@ -1535,8 +1535,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 			goto out;
 
 		if (copy_to_user(user_stack, stack,
-				 sizeof(struct kvm_ia64_vcpu_stack)))
+				 sizeof(struct kvm_ia64_vcpu_stack))) {
+			r = -EFAULT;
 			goto out;
+		}
 
 		break;
 	}