pstore/ram_core: Factor persistent_ram_zap() out of post_init()

A handy function that we will use outside of ram_core soon. But
so far just factor it out and start using it in post_init().

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>