kernel modules: add a quick scull port from LDD3

Also:

* fix fops.c on both kernels:
  * 5.9: the out of space error code was 1 not 8
  * 6.6: for whatever reason we can't read the user buffer as before on the
         diagnostic print, it leads to segfault and oops
* create memfile.c which is like fops.c but of unlimited size

rootfs_overlay/lkmc/seq_file.sh

@@ -1,7 +1,9 @@
 #!/bin/sh
 set -e
 f=/sys/kernel/debug/lkmc_seq_file
-insmod seq_file.ko
+mod="${1:-seq_file.ko}"
+shift
+insmod "$mod" "$@"
 [ "$(cat "$f")" = "$(printf '0\n1\n2\n')" ]
 [ "$(cat "$f")" = "$(printf '0\n1\n2\n')" ]
 [ "$(dd if="$f" bs=1 count=2 skip=0 status=none)" = "$(printf '0\n')" ]