Don't use st_gen in struct stat

[marcel]

Summary

Assign a random number to di_gen (for FFS), instead of extracting it from struct stat. We don't necessarily have permissions to see the generation number and the host OS may not have st_gen in struct stat anyway. Since the kernel assigns random numbers, there's nothing meaningful about the generation that requires us to preserve it when the file system image is created. So, simply assign di_gen by using random().

[marcel]

Self approve.