pf: Replace rwlock on PF_RULES_LOCK with rmlock

Given that PF_RULES_LOCK is a mostly read lock, replace the rwlock with rmlock.
This change improves packet processing rate in high pps environments.
Benchmarking by olivier@ shows a 65% improvement in pps.

While here, also eliminate all appearances of "sys/rwlock.h" includes since it
is not used anymore.

Submitted by: farrokhi@
Differential Revision: https://reviews.freebsd.org/D15502