mm/zpool: use prefixed module loading

To avoid potential format string expansion via module parameters,
do not use the zpool type directly in request_module() without a
format string. Additionally, to avoid arbitrary modules being loaded
via zpool API (e.g. via the zswap_zpool_type module parameter) add a
"zpool-" prefix to the requested module, as well as module aliases for
the existing zpool types (zbud and zsmalloc).

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