MDEV-27967 Assertion !buf_pool.any_io_pending() failed

The test innodb.log_file_size would occasionally crash in
a debug assertion failure in srv_prepare_to_delete_redo_log_file().
In the core dump, the assertion would hold.

buf_pool_t::any_io_pending(): Avoid dirty reads by acquiring
buf_pool.mutex. This function is only being invoked in debug builds,
so we do not care about any performance impact.

storage/innobase/include/buf0buf.h

@@ -2042,9 +2042,14 @@ class buf_pool_t
   buf_tmp_buffer_t *io_buf_reserve() { return io_buf.reserve(); }
 
   /** @return whether any I/O is pending */
-  bool any_io_pending() const
+  bool any_io_pending()
   {
-    return n_pend_reads || n_flush_LRU() || n_flush_list();
+    if (n_pend_reads)
+      return true;
+    mysql_mutex_lock(&mutex);
+    const bool any_pending{n_flush_LRU_ || n_flush_list_};
+    mysql_mutex_unlock(&mutex);
+    return any_pending;
   }
   /** @return total amount of pending I/O */
   ulint io_pending() const