selftests/lkdtm: Fix expected text for free poison

Freed memory poisoning can be tested a few ways, so update the expected
text to reflect the non-Oopsing alternative.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210623203936.3151093-4-keescook@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>