pf: pass struct pf_pdesc to pf_walk_option6() and pf_walk_header6()

This makes their argument list shorter. Also fix a bug where pf_walk_option6()
used the outer header in the pd2 case.
ok henning@ mikeb@

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, dfff4707a1
Sponsored by: Rubicon Communications, LLC ("Netgate")