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 9f9cf83, 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

Author: markjdb