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/memfile.sh

@@ -0,0 +1,51 @@
+#!/bin/sh
+set -e
+
+# Helpers
+odraw() (
+  od -A n -t x1 -v "$@" | tr -d '\n' | cut -c 2-
+)
+
+# Setup
+f=/sys/kernel/debug/lkmc_memfile
+mod="${1:-memfile.ko}"
+shift
+insmod "$mod" "$@"
+
+# Starts off empty
+[ -z "$(cat "$f")" ]
+
+# write and check it is there
+printf 12 > "$f"
+[ 12 = "$(cat "$f")" ]
+
+# Append and check that it is there
+printf 34 >> "$f"
+[ 1234 = "$(cat "$f")" ]
+
+# Restart
+printf 56 > "$f"
+[ 56 = "$(cat "$f")" ]
+
+# skip
+printf 1234 > "$f"
+[ 23 = "$(dd if="$f" bs=1 count=2 skip=1)" ]
+
+# seek
+printf 1234 > "$f"
+printf xy | dd bs=1 of="$f" seek=1 conv=notrunc
+[ 1xy4 = "$(cat "$f")" ]
+
+# seek past the end
+printf 1234 > "$f"
+printf xy | dd bs=1 of="$f" seek=6 conv=notrunc
+[ '31 32 33 34 00 00 78 79' = "$(odraw "$f")" ]
+
+# Allocate 1 GB for fun.
+dd if=/dev/zero of="$f" bs=1k count=1M
+[ '00 00' = "$(dd if="$f" bs=1 count=2 skip=500M | odraw)" ]
+
+# Teardown
+rmmod memfile
+
+echo passed