pf: Fix duplicate storage of direction

The variable storing the direction of a processed packet is passed
around to many functions. Most of those functions already have a pointer
to struct pf_pdesc which also contains the direction. By using the one
in struct pf_pdesc we can reduce the amount of arguments passed around.

Reviewed by: kp
Sponsored by: InnGames GmbH
Differential Revision: https://reviews.freebsd.org/D41008