diotest3: fix -f argument

The '-f' argument to diotest3 is currently ignored. During options
processing it's copied to the buffer, then subsequently overwritten
by the default in a later sprintf() call.

Move the setting of the default prior to the options processing.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>