pf: Factor out pf_krule_free()

Reviewed by: melifaro@
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29194

(cherry picked from commit 5e9dae8e149ae8848f52148b665f3a0d031ca40f)