pf: Ensure that we don't use kif passed to pfi_kkif_attach()

Once a kif is passed to pfi_kkif_attach() we must ensure we never re-use
it for anything else.
Set the kif to NULL afterwards to guarantee this.

Reported-by: syzbot+be5d4f4a7a4c295e659a@syzkaller.appspotmail.com
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 5e98cae661f3a7e85df7f277660765b823454073)