util: rm_rf_children_dangerous: delete all descendants dangerously

util: rm_rf_children_dangerous: delete all descendants dangerously

Call rm_rf_children_dangerous() recursively rather than falling back to
rm_rf_children(). This fixes a bug in systemd-tmpfiles.

The problem can easily be reproduced by:

  # mount /dev/sda1 /mnt
  # mkdir /mnt/test
  # echo "D /mnt" > /root/test.conf
  # systemd-tmpfiles --remove /root/test.conf
  Attempted to remove disk file system, and we can't allow that.
  rm_rf(/root/test): Operation not permitted

Reported-by: Lukas Jirkovsky <l.jirkovsky@gmail.com>