When ALTER TABLE is aborted, do not write garbage pages to data files

Fixing the potential corruption bug MDEV-14310 will reduce the performance of ALTER TABLE when the operation fails or is interrupted.
There is no reason to write pages that would be freed as part of rolling back the operation.

Maybe there should be a separate flush list for the pages for which redo logging has been disabled. Then the FlushObserver could simply leave buf_pool_t::flush_list alone.