fail on fallocate failure

[travisdowns]

fio should have a mode where when fallocating the file fails, it fails. Currently it seems that both fallocate=native (the default) and fallocate=posix both just don't fallocate the file if it fails, with the former option's behavior even being documented ("falls back to none on failure").

Silently continuing (posix isn't totally silent, but its easy to miss) leads to very strange results, especially when the configuration ends up exceeding the available disk space, since performance depends heavily on whether the allocation succeeded or not. So maybe something like die_on_fallocation_failure=1 would just hard stop if this occurs.