pf: fix SCTP SDT probe

We want the return value of pf_test_rule(), i.e. the result of the
evaluation of the new state, not the result of the evaluation of the
original packet/state.

MFC after: 1 week
Sponsored by: Orange Business Services

(cherry picked from commit b00dbe851c66f26a16219d31c76b7fb411ace94c)