kernel module: fix fops... and move its documentation into README

Sometimes I wonder if anyone has ever run this tutorial, otherwise how
can such basic bugs persist for so long?

test_all.sh: crete

rootfs_overlay/test_all.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+(
+  set -ex
+  /character_device.sh
+  /character_device_create.sh
+  /fops.sh
+)
+if [ "$?" -eq 0 ]; then
+  echo lkmc_test_pass
+  exit 0
+else
+  echo lkmc_test_fail
+  exit 1
+fi