zfs: remove duplicate ldflag

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: retrieve zfs dataset name from root directory

- check on which filesystem the directory '/var/lib/docker' is located
- if its a zfs get the dataset name from /proc/mounts

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: use local free() to reduce boilerplate code

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>