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/scull.sh

@@ -0,0 +1,59 @@
+#!/bin/sh
+set -eux
+
+name=scull
+
+mod="${1:-$name.ko}"
+shift
+insmod "$mod" "$@"
+major="$(awk "\$2==\"$name\" {print \$1}" /proc/devices)"
+
+rm -f /dev/${name}[0-3]
+mknod /dev/${name}0 c $major 0
+mknod /dev/${name}1 c $major 1
+mknod /dev/${name}2 c $major 2
+mknod /dev/${name}3 c $major 3
+
+rm -f /dev/${name}pipe[0-3]
+mknod /dev/${name}pipe0 c $major 4
+mknod /dev/${name}pipe1 c $major 5
+mknod /dev/${name}pipe2 c $major 6
+mknod /dev/${name}pipe3 c $major 7
+
+rm -f /dev/${name}single
+mknod /dev/${name}single c $major 8
+
+rm -f /dev/${name}uid
+mknod /dev/${name}uid c $major 9
+
+rm -f /dev/${name}wuid
+mknod /dev/${name}wuid c $major 10
+
+rm -f /dev/${name}priv
+mknod /dev/${name}priv c $major 11
+
+## scull
+
+f="/dev/${name}0"
+[ -z "$(cat "$f")" ]
+
+# Append starts writing from the start of the 4k block, not like the usual semantic.
+printf asdf > "$f"
+printf qw >> "$f"
+[ qwdf = "$(cat "$f")" ]
+
+# Overwrite first clears everything, then writes to start of 4k block.
+printf asdf > /dev/${name}0
+printf qw > /dev/${name}0
+[ qw = "$(cat "$f")" ]
+
+# Read from the middle
+printf asdf > /dev/${name}0
+[ df = "$(dd if="$f" bs=1 count=2 skip=2 status=none)" ]
+
+# Write to the middle
+printf asdf > "$f"
+printf we | dd of="$f" bs=1 seek=1 conv=notrunc status=none
+[ awef = "$(cat "$f")" ]
+
+echo passed