It turns out that we had some unnecessary waits for no outstanding
write requests to exist. They were basically working around a
bug that was fixed in MDEV-25953.

On write completion callback, blocks will be marked clean.
So, it is sufficient to consult buf_pool.flush_list to determine
which writes have not been completed yet.

On FLUSH TABLES...FOR EXPORT we must still wait for all pending
asynchronous writes to complete, because buf_flush_file_space()
would merely guarantee that writes will have been initiated.