Comment 37 for bug 1848856

On Ubuntu 19.10, with ZFS root and zfs-auto-snapshot, /etc/grub.d/10_linux_zfs was scanning every single snapshot whenever update-grub was invoked.

This is a simple patch that just exits the get_dataset_info() function if the dataset is a snapshot. I could be missing something, but I don't expect we want to be finding kernel & initrd images on snapshots under normal circumstances.