Server may hang when innodb_undo_log_truncate=ON

MDEV-27058 introduced a deadlock between trx_purge_truncate_history() and buf_pool_t::release_freed_page(). The former function is holding buf_pool.flush_list_mutex while waiting for a page X-latch:

    mysql_mutex_lock(&buf_pool.flush_list_mutex);

    for (buf_page_t *bpage= UT_LIST_GET_LAST(buf_pool.flush_list); bpage; )

    {

…

        bpage->lock.x_lock();

At the same time, buf_pool_t::release_freed_page() may hold a U-latch on this page while waiting for buf_pool.flush_list_mutex.

Before MDEV-27058, there was no problem here, because we would first buffer-fix the block, then release buf_pool.flush_list_mutex and only then acquire the block X-latch.

To fix this, we may invoke bpage->lock.x_lock_try() instead. If it fails, we may release and reacquire buf_pool.flush_list_mutex and restart the scan.

Because MDEV-27058 was not part of any release yet, this regression does not affect any release.