linux - How to shred already removed file on modern filesystem? (randomize free space, journal, tails, ... securely)

Consider the following:

mount | grep home
... type reiserfs ...

rm -Rf /home/user/over_9000_little_and_big_super_secret_files/

# oops, I should have shredded it instead.

How can I properly and securely "initialize free space" to ensure that no additional info can be restored restored by digging in free space? (Preferrably without stopping or disturbing the filesystem much.)

Is dd if=/dev/frandom of=/home/qqqqq really secure for this (tails, journal, etc.)?