pf: Sprinkle const qualifiers in state lookup routines

State keys are trivially const in lookup routines, so annotate them as
such. No functional change intended.

Reviewed by: kp
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Modirum
Differential Revision: https://reviews.freebsd.org/D45671

(cherry picked from commit 66b8cac8d837c0ca3fd38d0a66259ca932a1c430)