We call into softdep_prerename() and softdep_prelink() when there is low free space in the journal. Functions sync all vnodes participating in the VOP, in the hope that this would reduce journal utilization. But if the vnodes are already synced, doing sync would only spend writes, journal is filled not due to the records from modifications of our vnodes.
Remember original seqc numbers for vnodes, and only initiate syncs when seqc changed.
Tested by: pho