sd: set max filesize

bareos-sd.d/lto7.conf

@@ -9,7 +9,7 @@ Device {
   AlwaysOpen = yes;
   RemovableMedia = yes;
   RandomAccess = no;
-  #Maximum File Size = 50GB;
+  Maximum File Size = 20GB;
   AutomaticMount = yes;
   LabelMedia = yes;
 }