zfs dracut module should not assume systemd presence

Signed-off-by: Moritz Maxeiner <moritz@ucworks.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4749
Closes #5058

contrib/dracut/90zfs/mount-zfs.sh.in

@@ -10,6 +10,7 @@ case "${root}" in
 	*) return ;;
 esac
 
+if command -v systemctl >/dev/null; then
 	# If sysroot.mount exists, the initial RAM disk configured
 	# it to mount ZFS on root.  In that case, we bail early.
 	loadstate="$(systemctl --system --show -p LoadState sysroot.mount || true)"
@@ -19,6 +20,7 @@ else
 		info "ZFS: sysroot.mount present, delegating root mount to it"
 		return
 	fi
+fi
 
 # Delay until all required block devices are present.
 udevadm settle