csv plugin: fix forgotten "datadir" in re-implement value_list_to_filena...

In the commit 7f90e30 the re-implementation of value_list_to_filename forgot to prepend the datadir string.

Even if you put:

     DataDir "/var/lib/collectd/cvs"

in csv plugin config
you will get files like:

    /var/lib/collectd/localhost/load/load-2013-08-06

instead

    /var/lib/collectd/csv/localhost/load/load-2013-08-06

Notice that you are writing in basedir "/var/lib/collectd"
The version 5.3.1 is affected.