MDEV-30180 Server hang with innodb_undo_log_truncate=ON

trx_purge_truncate_history(): In case of an apparent conflict with
buf_pool_t::release_freed_page(), always momentarily acquire
buf_pool.mutex, to let the thread that is holding it and waiting for
buf_pool.flush_list_mutex to proceed.

The fix in commit c410f7a was
insufficient, because simple yielding would not necessarily let
the conflicting thread to acquire buf_pool.flush_list_mutex.

This hang was reported and the fix tested by Axel Schwenke.

storage/innobase/trx/trx0purge.cc

@@ -766,11 +766,12 @@ TRANSACTIONAL_TARGET static void trx_purge_truncate_history()
         auto block= reinterpret_cast<buf_block_t*>(bpage);
         if (!bpage->lock.x_lock_try())
         {
+        rescan:
           /* Let buf_pool_t::release_freed_page() proceed. */
           mysql_mutex_unlock(&buf_pool.flush_list_mutex);
-          std::this_thread::yield();
+          mysql_mutex_lock(&buf_pool.mutex);
           mysql_mutex_lock(&buf_pool.flush_list_mutex);
-        rescan:
+          mysql_mutex_unlock(&buf_pool.mutex);
           bpage= UT_LIST_GET_LAST(buf_pool.flush_list);
           continue;
         }