pstore/blk: Provide way to query pstore configuration

In order to configure itself, the MTD backend needs to be able to query
the current pstore configuration. Introduce pstore_blk_get_config() for
this purpose.

Signed-off-by: WeiXiong Liao <liaoweixiong@allwinnertech.com>
Link: https://lore.kernel.org/lkml/20200511233229.27745-9-keescook@chromium.org/


Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>