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

configure

@@ -109,11 +109,11 @@ fi
   # In particular:
   # - `shallow = true` on the submodule has no effect for the non default educational branches of our submodules
   # - QEMU's submodules point to commits that are neither under branches nor tags, and so `--shallow-submodules` fails
-  #
   git submodule update --depth 1 $gitjobs --init -- $submodules
   if "$qemu"; then
     cd qemu
     git submodule update --init --recursive
   fi
 ) &
+# https://unix.stackexchange.com/questions/65532/why-does-set-e-not-work-inside-subshells-with-parenthesis-followed-by-an-or
 wait $! || git submodule update --init --recursive -- $submodules