pf: Remove some state pointer indirection

Several subroutines take a pointer to a pointer to a pf state, but never
modify the input pointer. As in commit 9f9cf83f114a, let's remove the
indirection, making the code easier to read. No functional change
intended.

Reviewed by: kp, glebius
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D49519