Fix open() call with O_CREAT flag (mode parameter must be provided)

[agoncharuk]

When built on Ubuntu 21.04, the dbgen crashes complaining that open() is called with O_CREAT flag, but the mode parameter is not provided, which is in line with the open() documentation:

The mode argument must be supplied if O_CREAT or O_TMPFILE is specified in flags; if it is not supplied, some arbitrary bytes from the stack will be applied as the file mode.

The PR adds the required mode parameter in line with the similar calls below. After this change dbgen runs successfully on my system.