Make sure pflog is attached after pf is initializaed so we can
borrow pf's lock, and also make sure pflog goes after pf is gone
in order to avoid callouts in VNETs to an already freed instance.

Reported by: Ivan Klymenko, Johan Hendriks on current@ today
Obtained from: projects/vnet
Sponsored by: The FreeBSD Foundation
MFC after: 13 days
Approved by: re (gjb)