qsync: fix txn fiber hang on fencing at CONFIRM

If the limbo was fenced during CONFIRM WAL write, then the
confirmed txn was committed just fine, but its author-fiber kept
hanging. This is because when it was woken up, it checked if the
limbo is frozen and went to infinite waiting before actually
checking if the txn is completed.

The fiber would unfreeze if would be woken up explicitly as a
workaround.

The fix is simple - change the checks order.

Part of #7253

NO_DOC=bugfix

(cherry picked from commit ec628100)