HAMMER VFS - Fix serious bug when downgrading (and later upgrading) a PFS

HAMMER VFS - Fix serious bug when downgrading (and later upgrading) a PFS

* When downgrading a PFS master to a slave the sync-end-tid (in the
  pfs-status) is not updated.  This will cause the data to become
  inaccessible.

  Then, when upgrading back to a master the original stale sync-end-tid
  is used to rollback the PFS, effectively destroying its contents.

* We now properly update sync-end-tid when downgrading a PFS.  This
  makes the data accessible in slave mode and prevents the rollback
  when re-upgrading the PFS from destroying any of the master's original
  data.

* Why is a rollback used at all when upgrading you ask?  When a PFS is
  operating as a slave mirroring operations can be interrupted, leaving
  a lot of partially updated records.  Since sync-end-tid is not updated
  until the mirroring operation completes (when in slave mode), these
  partially mirrored records are not visible.

  However, if the slave is upgraded to a master any records from
  incomplete mirroring operations must be destroyed.  Hence the rollback
  during an upgrade is a necessary feature under normal operation.

Reported-by: Thomas Nikolajsen