MDEV-31350: Hang in innodb.recovery_memory

buf_flush_page_cleaner(): Whenever buf_pool.ran_out(), invoke
buf_pool.get_oldest_modification(0) so that all clean blocks
will be removed from buf_pool.flush_list and buf_flush_LRU_list_batch()
will be able to evict some pages.

This fixes a regression that was likely caused by
commit a55b951e (MDEV-26827).

storage/innobase/buf/buf0flu.cc

@@ -2330,6 +2330,7 @@ static void buf_flush_page_cleaner()
     else if (buf_pool.ran_out())
     {
       buf_pool.page_cleaner_set_idle(false);
+      buf_pool.get_oldest_modification(0);
       mysql_mutex_unlock(&buf_pool.flush_list_mutex);
       n= srv_max_io_capacity;
       mysql_mutex_lock(&buf_pool.mutex);