vhost_net: add a missing error return

We accidentally left out this error return so it leads to some use after
free bugs later on.

Fixes: 0a0be13b ("vhost_net: batch submitting XDP buffers to underlayer sockets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>