pstore/ram: Move internal definitions out of kernel-wide include

Most of the details of the ram backend are entirely internal to the
backend itself. Leave only what is needed to instantiate a ram backend
in the kernel-wide header.

Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Colin Cross <ccross@android.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-and-tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Link: https://lore.kernel.org/r/20221011200112.731334-4-keescook@chromium.org