pstore/ram_core: Proper checking for post_init errors (e.g. improper ECC size)

We will implement variable-sized ECC buffers soon, so post_init routine
might fail much more likely, so we'd better check for its errors.

To make error handling simple, modify persistent_ram_free() to it be safe
at all times.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>