pf: remove ip(6) header argument from pf_reassemble(6)()

Instead of passing the ip header and mbuf to pf_reassemble(), lookup
the header address in the mbuf.
ok henning@

Reviewed by: zlei
Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, 074ee1f915
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46652