graphdriver/zfs: fix GetMetadata() comment

Commit e27c904 added a wrong and misleading comment
to GetMetadata(). Fix it using the wording from
commit 407a626 which introduced GetMetadata().

Signed-off-by: Kir Kolyshkin <kir@openvz.org>

daemon/graphdriver/zfs/zfs.go

@@ -200,7 +200,7 @@ func (d *Driver) Status() [][2]string {
 	}
 }
 
-// GetMetadata is used for implementing the graphdriver.ProtoDriver interface. ZFS does not currently have any meta data.
+// GetMetadata returns image/container metadata related to graph driver
 func (d *Driver) GetMetadata(id string) (map[string]string, error) {
 	return nil, nil
 }