pf: rename pf_state to pf_kstate

Indicate that this is a kernel-only structure, and make it easier to
distinguish from others used to communicate with userspace.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31096

(cherry picked from commit 211cddf9e3a1bc0d4b1b94bea7d16a47b5a17f49)