aggressive; rework to be a bit less susceptable to round-off error.
now it's likely that the density might not be obtained with a small
filesystem with a large number of inodes (e.g -s 4M -i 1k), but that's
an extremely unlikely corner case that can easily be rectified with
command-line arguments.
fixed provided in private email by Takao Shinohara <shin@sm.sony.co.jp>
should resolve PRs [bin/14049] and [bin/14046]
058b690102