pf: Always skip outbound filtering for inbound af-to rules

The af-to rules on inbound direction create a single state spanning both
the inbound and the outbound interface. Calling pf_test() for the
outbound direction in pf_route() makes the packet pass through state
search, match the existing state, never evaluate the ruleset, and increase
state counters.

Check that the state comes from an af-to rule in inbound direction, and
if yes, skip outbound testing.

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D52446