zfs: fix LINUX_ROOT_DEVICE when grub-probe fails

zfs: fix LINUX_ROOT_DEVICE when grub-probe fails

When grub-probe fails, the current code is to just stuff an empty result
in which causes the user to not knowingly have a system that no longer
boots. grub-probe can fail because the ZFS pool that contains the root
filesystem might have features that grub does not yet support which is a
common configuration for people with a rpool and a bpool. This behavior
uses the zdb utility to dump the same value as the filesystem label
would print.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Gbp-Pq: Topic upstream
Gbp-Pq: Name zfs-fix-LINUX_ROOT_DEVICE-when-grub-probe-fails.patch