Notify that the ifnet will go away, even on vnet shutdown

pf subscribes to ifnet_departure_event events, so it can clean up the
ifg_pf_kif and if_pf_kif pointers in the ifnet.
During vnet shutdown interfaces could go away without sending the event,
so pf ends up cleaning these up as part of its shutdown sequence, which
happens after the ifnet has already been freed.

Send the ifnet_departure_event during vnet shutdown, allowing pf to
clean up correctly.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D17500