pstore/ram: Improve backward compatibility with older Chromebooks

pstore/ram: Improve backward compatibility with older Chromebooks

When you try to run an upstream kernel on an old ARM-based Chromebook
you'll find that console-ramoops doesn't work.

Old ARM-based Chromebooks, before <https://crrev.com/c/439792>
("ramoops: support upstream {console,pmsg,ftrace}-size properties")
used to create a "ramoops" node at the top level that looked like:

/ {
  ramoops {
    compatible = "ramoops";
    reg = <...>;
    record-size = <...>;
    dump-oops;
  };
};

...and these Chromebooks assumed that the downstream kernel would make
console_size / pmsg_size match the record size.  The above ramoops
node was added by the firmware so it's not easy to make any changes.

Let's match the expected behavior, but only for those using the old
backward-compatible way of working where ramoops is right under the
root node.

NOTE: if there are some out-of-tree devices that had ramoops at the
top level, left everything but the record size as 0, and somehow
doesn't want this behavior, we can try to add more conditions here.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>