This change makes sure that in a case we try to do fast reboot using reboot(1m)
and we do not specify the target BE (using -e), but we do specify ZFS
filesystem (or plain disk) in boot_arguments, the operation is successful.

Without the fix:

# beadm create new
Created successfully
# reboot -f -- rpool/ROOT/new
reboot: rpool/ROOT/new is not a root disk or dataset
# beadm destroy -fF new
#

With the fix:

# beadm create new
Created successfully
# reboot -f -- rpool/ROOT/new
Connection to localhost closed by remote host.
Connection to localhost closed.
$