vhost-net: avoid flush under lock

We flush under vq mutex when changing backends.
This creates a deadlock as workqueue being flushed
needs this lock as well.

https://bugzilla.redhat.com/show_bug.cgi?id=612421

Drop the vq mutex before flush: we have the device mutex
which is sufficient to prevent another ioctl from touching
the vq.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -534,11 +534,16 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 	rcu_assign_pointer(vq->private_data, sock);
 	vhost_net_enable_vq(n, vq);
 done:
+	mutex_unlock(&vq->mutex);
+
 	if (oldsock) {
 		vhost_net_flush_vq(n, index);
 		fput(oldsock->file);
 	}
 
+	mutex_unlock(&n->dev.mutex);
+	return 0;
+
 err_vq:
 	mutex_unlock(&vq->mutex);
 err: