resolve zfs root identification

Grub fails to generate a configuration for debian on a zfs-root for quite some time now, and always requires a manual workaround. This bug report and the ones on grub have various patch suggestions, I took one from the debian bug added it and tested it. The generated grub.cfg now contains the correct drive name, as intended. I think this should resolve the corresponding bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=848945