In commit 685d958e (MDEV-14425)
we ended up not enabling O_DIRECT writes on the redo log
by default, because back then, it was slightly slower on
some systems.

With commit a635c406 (MDEV-27774)
the situation changed. A new test on a NVMe device shows 9%
improvement in throughput and over 15% reduction of latency
when O_DIRECT writes are enabled.

With this change, all the following settings will use O_DIRECT
on InnoDB data and log files:

innodb_flush_method=O_DIRECT
innodb_flush_method=O_DIRECT_NO_FSYNC
innodb_flush_method=O_DSYNC

Before MDEV-14425, log writes were always buffered on Linux.
Between MDEV-14425 and this change, unbuffered log writes
were only enabled for innodb_flush_method=O_DSYNC.