trx_purge_truncate_history(): Release buf_pool.flush_list_mutex
and 'dummily' acquire and release buf_pool.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 5dbe7a8c (MDEV-32757).

Tested by: Axel Schwenke (on Ubuntu 18.04 and Ubuntu 20.04)
Reviewed by: Vladislav Lesin