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

kernel_modules.code-workspace

@@ -0,0 +1,37 @@
+// This workspace exists to work on C files formatted like the Linux kernel,
+// notably using tabs instead of space. This is unlike the C files in our userland
+// programs, and we couldn't find a better way to make this distinction
+// https://stackoverflow.com/questions/47405315/visual-studio-code-and-subfolder-specific-settings
+{
+    "folders": [
+        {
+            "path": "."
+        },
+        {
+            "path": "submodules/linux"
+        }
+    ],
+    "settings": {
+        "files.watcherExclude": {
+            "data/**": true,
+            ".git/**": true,
+            "out.docker/**": true,
+            "out/**": true,
+            "submodules/**": true,
+        },
+        "search.exclude": {
+            "data/**": true,
+            ".git/**": true,
+            "out.docker/**": true,
+            "out/**": true,
+            "submodules/**": true,
+        },
+        "[c]": {
+            "editor.tabSize": 8,
+            "editor.insertSpaces": false
+        },
+        "files.associations": {
+            "rwsem.h": "c"
+        }
+    }
+}