pstore/ram: Move initialization earlier

Since only one single ramoops area allowed at a time, other probes
(like device tree) are meaningless, as it will waste CPU resources.
So let's check for being already initialized first.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Signed-off-by: Kees Cook <keescook@chromium.org>