pf: simplify state key setup

Just use pd->sidx and pd->didx to reverse the state key arguments
in pf_test_state_*(). This needs less code and is more consistent.
OK mikeb, markus, henning.

Obtained from: OpenBSD, mpf <mpf@openbsd.org>, 2c0d332046
Sponsored by: Rubicon Communications, LLC ("Netgate")