coccinelle: filter out a couple of 'false-positive' transformations

coccinelle: filter out a couple of 'false-positive' transformations

  * flag-set.cocci: perform the transformation only if the second
    argument is a constant
  * sd-journal/lookup3.c: skip the cocci completely for this file, since
    it's not "ours"
  * strjoina.cocci: skip the transformation on the "test_strjoina" test,
    since it intentionally tests the "incorrect" expression we're trying to
    transform (the same thing was already done in strjoin.cocci)