cherry-pick improved fallocate explanation by @drawks
cherry-pick improved fallocate explanation by @drawks

obfuscurity/carbon,criteo-forks/carbon,krux/carbon,krux/carbon,obfuscurity/carbon,deniszh/carbon,iain-buclaw-sociomantic/carbon,iain-buclaw-sociomantic/carbon,piotr1212/carbon,graphite-project/carbon,piotr1212/carbon,johnseekins/carbon,graphite-project/carbon,criteo-forks/carbon,deniszh/carbon,johnseekins/carbon