pf: Refactor state killing

Extract the state killing code from pfioctl() and rephrase the filtering
conditions for readability.

No functional change intended.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29795

(cherry picked from commit 586aab9e0aa6c811758c19fb03831fc1e7305252)