If innodb_flush_method=O_DSYNC, log_sys.flushed_to_disk_lsn  is changed
without 'flush_lock' protection inside log_write().

This leads to a race condition, if there are 2 threads running in parallel,
doing log_write_up_to() with different values for 'flush_to_disk'

In this case, log_write() and log_write_flush_to_disk_low() can execute at
the same time, and both would change flushed_lsn.

The fix is to remove special treatment of durable writes from log_write().
There is no apparent reason for this special treatment, log_write_flush_to_disk_low()
is already optimized for durable writes.

Nor there is an apparent reason to call log_flush_notify() more often in
for O_DSYNC.