Fix two logging messages in TLReplaceDisks

Commit f0f8d060 (“Show old primary/secondary node on disk
replacement”) added two wrong uses of feedback_fn, which results in
log entries such like these:

      "log": [
        [
          7, [1351258326, 466214], "message", "Replacing disk(s) 0 for instance 'instance1.example.com'"
        ],
        [
          8, [1351258326, 749391], "Current primary node: %s", "node1"
        ],
        [
          9, [1351258326, 774829], "Current seconary node: %s", "node4"
        ],
        [
          10, [1351258329, 620410], "message", "STEP 1/6 Check device existence"
        ],

(the first and fourth are OK, the 2nd and 3rd are broken).

The API of jqueue._OpExecCallbacks.Feedback should probably be changed
to prevent such an occurrence in the future.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>