As part of MDEV-23855, we eliminated fil_system.LRU
and changed the way how InnoDB data files are opened.
We are also enforcing the innodb_open_files limit when new
data files are created.

The function fil_space_t::flush() would be invoked by
row_quiesce_table_start(). If the table was already in clean state,
it is possible that the data file is not open.

fil_space_t::flush_low(): If the data file is not open,
check with a debug assertion that there are no unflushed changes,
and carry on.

Reviewed by: Eugene Kosov and Thirunarayanan Balathandayuthapani