pf: avoid passing through dummynet multiple times

In some setups we end up with multiple states created for a single
packet, which in turn can mean we run the packet through dummynet
multiple times. That's not expected or intended. Mark each packet when
it goes through dummynet, and do not pass packet through dummynet if
they're marked as having already passed through.

See also: https://redmine.pfsense.org/issues/14854
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D44365