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Before closing file handles, we really must wait until there are no
pending os_file_flush(). This was missed in
commit b1ab211d (MDEV-15053)
where we changed the following:

    fil_node_close_to_free(): Wait for n_pending==0. Because we no longer
    do an extra lookup of the tablespace between fil_io() and the
    completion of the operation, we must give fil_node_t::complete_io() a
    chance to decrement the counter.