Non-blocking log_write_upto (MDEV-24341) was only designed  for the
client connections. Fix, so it is not be triggered for any system THD.

Previously, an incomplete solution only excluded Innodb purge THDs, but
not  the slave for example.

The hang in MDEV still remains somewhat a mystery though, it is not
immediately clear how exactly condition variable can become corrupted.
But it is clear that it can be avoided.