pf: also apply dummynet to route-to/dup-to packets

If packets are processed by a route-to/dup-to/reply-to rule (i.e. they
pass through pf_route(6)) dummynet was not applied to them.
This is because pf_route(6) passes packets directly to ifp->if_output(),
so the dummynet functions were never called.

Factor out the dummynet code and call dummynet prior to
ifp->if_output(). This has a secondary benefit of reducing some code
duplication between the IPv4 and IPv6 paths.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35158