When we map a file for generating the DAG, we do a simple truncate to e.g. 1Gb. This is fine, even if we have nowhere near 1Gb disk available, as the actual file doesn't take up the full 1Gb, merely a few bytes. When we start generating into it, however, it eventually crashes with a unexpected fault address .

This change fixes it (on linux systems) by using the Fallocate syscall, which preallocates suffcient space on disk to avoid that situation. 
Co-authored-by: Felix Lange <fjl@twurst.com>