When a ng_tty node receives a byte or packet from the device, it sends
the data up through its hook. Once it makes its way up to an ng_iface
node, it gets dispatched through netisr, so the thread must have a VNET
By default ng_tty will use direct dispatch, i.e., the thread calling the
TTY hook will pass packets all the way up to netisr. However, it does
not set a VNET context, so in this case we crash.
Work around the problem by setting the current VNET before calling