pstore: migrate to crypto acomp interface

The crypto 'compress' interface is deprecated, so before adding new
features, migrate to the acomp interface. Note that we are only using
synchronous implementations of acomp, so we don't have to deal with
asynchronous completion.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>