trx_purge_truncate_history(): Release buf_pool.flush_list_mutex
before starting a rescan of buf_pool.flush_list, to ensure that
the buf_flush_page_cleaner thread (which may be holding buf_pool.mutex)
will be able to proceed.

This fixes up commit a0f02f74 (MDEV-32757).

Tested by: Axel Schwenke
Reviewed by: Vladislav Lesin