pf: Small performance tweak

Because fetching a counter is a rather expansive function we should use
counter_u64_fetch() in pf_state_expires() only when necessary. A "rdr
pass" rule should not cause more effort than separate "rdr" and "pass"
rules. For rules with adaptive timeout values the call of
counter_u64_fetch() should be accepted, but otherwise not.

From the man page:

The adaptive timeout values can be defined both globally and for
each rule.  When used on a per-rule basis, the values relate to the
number of states created by the rule, otherwise to the total number
of states.

This handling of adaptive timeouts is done in pf_state_expires(). The
calculation needs three values: start, end and states.

1. Normal rules "pass .." without adaptive setting meaning "start = 0" runs in the else-section and therefore takes "start" and "end" from the global default settings and sets "states" to pf_status.states (= total number of states).

2. Special rules like "pass .. keep state (adaptive.start 500 adaptive.end 1000)" have start != 0, run in the if-section and take "start" and "end" from the rule and set "states" to the number of states created by their rule using counter_u64_fetch().

Thats all ok, but there is a third case without special handling in the
above code snippet:

3. All "rdr/nat pass .." statements use together the pf_default_rule. Therefore we have "start != 0" in this case and we run the if-section but we better should run the else-section in this case and do not fetch the counter of the pf_default_rule but take the total number of states.

Submitted by: Andreas Longwitz <longwitz@incore.de>
MFC after: 2 weeks