Fixed a bug which causes the script to fail where it should succeed

First of all thanks for making this script. I've been using it for years now and it has served me well.

I just happened to do some new things with it (pack it in a container) and ran accross a bug I think was introduced recently. See patch.

is_on_root_fs is true when target is on root fs, and in that case the process should die. The ! was here not in place.