vhost: synchronize IOTLB message with dev cleanup

vhost: synchronize IOTLB message with dev cleanup

DaeRyong Jeong reports a race between vhost_dev_cleanup() and
vhost_process_iotlb_msg():

Thread interleaving:
CPU0 (vhost_process_iotlb_msg)			CPU1 (vhost_dev_cleanup)
(In the case of both VHOST_IOTLB_UPDATE and
VHOST_IOTLB_INVALIDATE)

=====						=====
						vhost_umem_clean(dev->iotlb);
if (!dev->iotlb) {
	        ret = -EFAULT;
		        break;
}
						dev->iotlb = NULL;

The reason is we don't synchronize between them, fixing by protecting
vhost_process_iotlb_msg() with dev mutex.

Reported-by: DaeRyong Jeong <threeearcat@gmail.com>
Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>