If the execution of the two reads in log_t::get_lsn_approx() is
interleaved with concurrent writes of those fields in
log_t::write_buf() or log_t::persist(), the returned approximation
will be an upper bound. If log_t::append_prepare_wait() is pending,
the approximation could be a lower bound.
We must adjust each caller of log_t::get_lsn_approx() for the
possibility that the return value is larger than
MAX(oldest_modification) in buf_pool.flush_list.
af_needed_for_redo(): Add a comment that explains why the glitch
is not a problem.
page_cleaner_flush_pages_recommendation(): Revise the logic for
the unlikely case cur_lsn < oldest_lsn. The original logic would have
invoked af_get_pct_for_lsn() with a very large age value, which
would likely cause an overflow of the local variable lsn_age_factor,
and make pct_for_lsn a "random number". Based on that value,
total_ratio would be normalized to something between 0.0 and 1.0.
Nothing extremely bad should have happened in this case;
the innodb_io_capacity_max should not be exceeded.
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