Once pf became not covered by a single mutex, many counters in it became
race prone. Some just gather statistics, but some are later used in
different calculations.

A real problem was the race provoked underflow of the states_cur counter
on a rule. Once it goes below zero, it wraps to UINT32_MAX. Later this
value is used in pf_state_expires() and any state created by this rule
is immediately expired.

Thus, make fields states_cur, states_tot and src_nodes of struct
pf_rule be counter(9)s.

Thanks to Dennis for providing me shell access to problematic box and
his help with reproducing, debugging and investigating the problem.

Thanks to:		Dennis Yusupoff <dyr smartspb.net>
Also reported by:	dumbbell, pgj, Rambler
Sponsored by:		Nginx, Inc.