pf: remove unneeded double pointer to pf_scrub_ip(6)()

pf_scrub_ip() does not modify the given mbuf pointer. So don't
pass a pointer to a pointer to make the code in pf_test() clearer.
ok henning@

Reviewed by: zlei
Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, aac78b59b9
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46593