pf: move the mbuf into struct pf_pdesc too

As requested by henning, move the mbuf pointer into struct pf_pdesc.
Also sort pd to the beginning of the functions' parameter lists for
consistency.
ok henning

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 776f210a75
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46941