pf: allow table stats clearing and reading with ruleset rlock

Instead serialize against these operations with a dedicated lock.

Prior to the change, When pushing 17 mln pps of traffic, calling
DIOCRGETTSTATS in a loop would restrict throughput to about 7 mln. With
the change there is no slowdown.

Reviewed by: kp (previous version)
Sponsored by: Rubicon Communications, LLC ("Netgate")