KVM: return an error code in kvm_vm_ioctl_register_coalesced_mmio()

If kvm_io_bus_register_dev() fails then it returns success but it should
return an error code.

I also did a little cleanup like removing an impossible NULL test.

Cc: stable@vger.kernel.org
Fixes: 2b3c246a682c ('KVM: Make coalesced mmio use a device per zone')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/coalesced_mmio.c

@@ -154,17 +154,13 @@ int kvm_vm_ioctl_register_coalesced_mmio(struct kvm *kvm,
 	list_add_tail(&dev->list, &kvm->coalesced_zones);
 	mutex_unlock(&kvm->slots_lock);
 
-	return ret;
+	return 0;
 
 out_free_dev:
 	mutex_unlock(&kvm->slots_lock);
-
 	kfree(dev);
 
-	if (dev == NULL)
-		return -ENXIO;
-
-	return 0;
+	return ret;
 }
 
 int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm *kvm,