debugfs.c: extend example with toplevel file, fop and kstrtoull_from_user

rootfs_overlay/debugfs.sh

@@ -1,12 +1,22 @@
 #!/bin/sh
 set -ex
-mkdir -p /debugfs
+d=/debugfs
+mkdir -p "$d"
 # We also added a fstab entry that mounts this under /sys/kernel/debug autmoatically.
 # That is the most common place to mount it.
 # The /sys/kernel/debug directory gets created automatically when debugfs is
 # compiled into the kernel, but it does not get mounted automatically.
 mount -t debugfs none /debugfs
 insmod /debugfs.ko
-cd /debugfs/lkmc_debugfs
+cd "${d}/lkmc_debugfs"
+
 cat myfile
 # => 42
+
+echo 13 > myfile
+cat myfile
+# => 13
+
+echo 666 > "${d}/lkmc_debugfs_file"
+cat myfile
+# => 666