vdpa_sim: avoid putting an uninitialized iova_domain

The system will crash if we put an uninitialized iova_domain, this
could happen when an error occurs before initializing the iova_domain
in vdpasim_create().

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
RIP: 0010:__cpuhp_state_remove_instance+0x96/0x1c0
...
Call Trace:
 <TASK>
 put_iova_domain+0x29/0x220
 vdpasim_free+0xd1/0x120 [vdpa_sim]
 vdpa_release_dev+0x21/0x40 [vdpa]
 device_release+0x33/0x90
 kobject_release+0x63/0x160
 vdpasim_create+0x127/0x2a0 [vdpa_sim]
 vdpasim_net_dev_add+0x7d/0xfe [vdpa_sim_net]
 vdpa_nl_cmd_dev_add_set_doit+0xe1/0x1a0 [vdpa]
 genl_family_rcv_msg_doit+0x112/0x140
 genl_rcv_msg+0xdf/0x1d0
 ...

So we must make sure the iova_domain is already initialized before
put it.

In addition, we may get the following warning in this case:
WARNING: ... drivers/iommu/iova.c:344 iova_cache_put+0x58/0x70

So we must make sure the iova_cache_put() is invoked only if the
iova_cache_get() is already invoked. Let's fix it together.

Cc: stable@vger.kernel.org
Fixes: 4080fc10 ("vdpa_sim: use iova module to allocate IOVA addresses")
Signed-off-by: Longpeng <longpeng2@huawei.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211124015215.119-1-longpeng2@huawei.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -591,8 +591,11 @@ static void vdpasim_free(struct vdpa_device *vdpa)
 		vringh_kiov_cleanup(&vdpasim->vqs[i].in_iov);
 	}
 
+	if (vdpa_get_dma_dev(vdpa)) {
 		put_iova_domain(&vdpasim->iova);
 		iova_cache_put();
+	}
+
 	kvfree(vdpasim->buffer);
 	if (vdpasim->iommu)
 		vhost_iotlb_free(vdpasim->iommu);