pf: simplify action resolution in pf_test_rule()

Rather than looking at state keys after the fact just save the action in the
nat64 section. This simplifies things and it also ensures we don't use state
keys after pf_insert_state().

Reviewed by: glebius, markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D49552