innodb_undo_log_truncate is not crash-safe

Likely caused by MDEV-18976, innodb_undo_log_truncate=ON lost its crash-safety status. The reason is that the pages of the being-truncated undo log tablespace were not marked as modified by the mini-transaction. The logic of logging page-free operations was changed in MDEV-18976.

The failures are rare and hard to repeat, but there is some indication that a small fix to trx_purge_free_segment() would fix this.

I did not find any matching failures of the test innodb.undo_log_truncate between 10.3 and 10.5.