dmsquash-live/iso-scan: Provide an easy reference to iso-scan device.

With commit 3c8c807, /run/initramfs/isoscan and /run/initramfs/live
mountpoints are unmounted upon rd.live.ram boots.  Save a link to
the iso-scan device in /run/initramfs/isoscandev to easily remount
the source, if desired.

modules.d/90dmsquash-live/iso-scan.sh

@@ -22,6 +22,7 @@ do_iso_scan() {
         mount -t auto -o ro "$dev" "/run/initramfs/isoscan" || continue
         if [ -f "/run/initramfs/isoscan/$isofile" ]; then
             losetup -f "/run/initramfs/isoscan/$isofile"
+            ln -s $dev /run/initramfs/isoscandev
             rm -f -- "$job"
             exit 0
         else