pf: ensure we don't destroy an uninitialised lock

The new lock introduced in 5f5e32f1b3 needs to be initialised early so
that it can be safely destroyed if we error out.

Reported-by: syzbot+d76113e9a4ae0c0fcac2@syzkaller.appspotmail.com
MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit e5ca5e801d3c806a767c427360c48e935a5facd4)