pf: replace union pf_krule_ptr with struct pf_krule in in-kernel structs

There is no need for the union pf_krule_ptr for kernel-only structs like
pf_kstate and pf_ksrc_node. The rules are always accessed by pointer. The rule
numbers are a leftover from using the same structure for pfctl(8) and pf(4).

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D46868