pf: Split pf_src_node into a kernel and userspace struct

Introduce a kernel version of struct pf_src_node (pf_ksrc_node).

This will allow us to improve the in-kernel data structure without
breaking userspace compatibility.

Reviewed by: philip
MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27707