Under some pathological 32-bit configs, the shift overflow KUnit tests
create huge stack frames. Split up the function to avoid this,
separating by rough shift overflow cases.

Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Daniel Latypov <dlatypov@google.com>
Cc: Vitor Massaru Iha <vitor@massaru.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202208301850.iuv9VwA8-lkp@intel.comAcked-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>