Prevent race between pf_test() and pf_purge_expired_states().

Prevent race between pf_test() and pf_purge_expired_states().
Packets (callers to pf_test()) must alter pf_state::timeout
under protection of pf_state::mtx. We also have to make sure
the packet does not update pf_state::timeout when ::timeout
reaches PFTM_UNLINKED.

The first report came from Johan Huldtgren, but he is not
the single user who has noticed "st->timeout == PFTM_UNLINKED"
assert violation.

OK bluhm@