vhost: fix error path in vhost_net_set_backend

An error could cause vhost_net_set_backend to exit without unlocking
vq->mutex. Fix this.
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -508,12 +508,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 	/* Verify that ring has been setup correctly. */
 	if (!vhost_vq_access_ok(vq)) {
 		r = -EFAULT;
-		goto err;
+		goto err_vq;
 	}
 	sock = get_socket(fd);
 	if (IS_ERR(sock)) {
 		r = PTR_ERR(sock);
-		goto err;
+		goto err_vq;
 	}
 
 	/* start polling new socket */
@@ -524,12 +524,14 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 	vhost_net_disable_vq(n, vq);
 	rcu_assign_pointer(vq->private_data, sock);
 	vhost_net_enable_vq(n, vq);
-	mutex_unlock(&vq->mutex);
 done:
 	if (oldsock) {
 		vhost_net_flush_vq(n, index);
 		fput(oldsock->file);
 	}
+
+err_vq:
+	mutex_unlock(&vq->mutex);
 err:
 	mutex_unlock(&n->dev.mutex);
 	return r;