There is relationship between LOCK_flush and LOCK_open. Even though it is not on a hot path, it indirectly interferes with TABLE_SHARE::tdc.flushed. The latter is on hot path and will eventually be moved out of LOCK_open.
The sole purpose of LOCK_flush is to let tdc_remove_table() wait for concurrent FLUSH TABLES. Replace global rwlock with per-share condition variable.