pfsync: Ensure uninit is done before pf

[kp]

Summary

pfsync touches pf memory (for pf_state and the pfsync callback
pointers), not the other way around. We need to ensure that pfsync is
torn down before pf.

[bz]

I assume the "other way round" is only true after the function pointers have been virtualised, otherwise not.
And that makes me wonder..

[bz]

OK, this only makes sense after the callback functions are virtualized; before the above comment did make sense but was expecting a different architechtural solution (whether true or not at the time of writing)