trx_purge_truncate_history(): Avoid a deadlock with
buf_pool_t::release_freed_page(). Page latches are not supposed
to be waited for while holding a mutex like buf_pool.mutex or
buf_pool.flush_list_mutex.

This regression was caused by
commit aaef2e1d (MDEV-27058).
Before that, trx_purge_truncate_history() would buffer-fix the block,
release buf_pool.flush_list_mutex, and then wait for the
exclusive page latch.

This bug led to occasional failures of the test
innodb.undo_truncate_recover.