pf: add ttl to pf_pdesc

This simplifies the code a little, because we can now avoid looking at the
IP(v6) header at all in pf_test() itself.

We do have to move the Jumbogram check into pf_setup_pdesc as well.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46928