This had been originally added in
mysql/mysql-server@192bb153b675fe09037a53e456a79eee7211e3a7
with the motivation to disable O_DIRECT for the dedicated tablespace
for temporary tables. In MariaDB Server,
commit 5eb53955 (MDEV-12227)
should be a better solution.

The code became orphaned later in
mysql/mysql-server@c61244c0e6c58727cffebfb312ac415a463fa0fe
and it had been applied to MariaDB Server 10.2.2 in
commit 2e814d47 and
commit fec844ac.

Thanks to Vladislav Vaintroub for spotting this.