pf: fix dummynet + NAT

Dummynet differs from ALTQ in that ALTQ schedules packets after they
leave pf. Dummynet schedules them after they leave pf, but then
re-injects them.
We currently deal with this by ensuring we don't re-schedule a packet we
get from dummynet, but this produces unexpected results when combined
with NAT, as dummynet processing is done after the NAT transformation.
In other words, the second time the packet is handed to pf it may have a
different source and destination address.

Simplify this by moving dummynet processing to after all other pf
processing, and not re-processing (but always passing) packets from
dummynet.

This fixes NAT of dummynet delayed packets, and also reduces processing
overhead (because we only do state/rule lookup for each dummynet packet
once, rather than twice).

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32665