strip block: off root= when using the mount hooks

I'm not sure they are still used, but if so they need to
trim the prefix.

modules.d/95rootfs-block/mount-root.sh

@@ -1,4 +1,4 @@
 #!/bin/sh
-if [ ! -s /.resume -a "$root" ]; then
+if [ ! -s /.resume -a -n "$root" -a -z "${root%%block:*}" ]; then
-    mount ${fstype:--t auto} -o "$rflags" "$root" "$NEWROOT" && ROOTFS_MOUNTED=yes
+    mount ${fstype:--t auto} -o "$rflags" "${root#block:}" "$NEWROOT" && ROOTFS_MOUNTED=yes
 fi