vhost: fix IOTLB locking

Commit 78139c94dc8c ("net: vhost: lock the vqs one by one") moved the vq
lock to improve scalability, but introduced a possible deadlock in
vhost-iotlb. vhost_iotlb_notify_vq() now takes vq->mutex while holding
the device's IOTLB spinlock. And on the vhost_iotlb_miss() path, the
spinlock is taken while holding vq->mutex.

Since calling vhost_poll_queue() doesn't require any lock, avoid the
deadlock by not taking vq->mutex.

Fixes: 78139c94dc8c ("net: vhost: lock the vqs one by one")
Acked-by: Jason Wang <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Jean-Philippe Brucker <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/vhost/vhost.c

···
 		if (msg->iova <= vq_msg->iova &&
 		    msg->iova + msg->size - 1 >= vq_msg->iova &&
 		    vq_msg->type == VHOST_IOTLB_MISS) {
-			mutex_lock(&node->vq->mutex);
 			vhost_poll_queue(&node->vq->poll);
-			mutex_unlock(&node->vq->mutex);
-
 			list_del(&node->node);
 			kfree(node);
 		}