In the backport of Bug#24450908 UNDO LOG EXISTS AFTER SLOW SHUTDOWN
from MySQL 5.7 to the MySQL 5.6 based MariaDB Server 10.1, we must
use a mutex when HAVE_ATOMIC_BUILTINS is not defined.

Also, correct a function comment. In MySQL 5.6 and MariaDB Server 10.1,
also temporary InnoDB tables are redo-logged.