lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()

At least on powerpc with GCC 6, the compiler is smart enough to optimise
lkdtm_CORRUPT_STACK() into an empty function that just returns.

If we print the buffer after we've written to it that prevents the
compiler from optimising away data and the memset().

Change-Id: I54403b905c8204ad87caacca8c86e554965cedbd
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: c55d240003ae253d3057dcf93510c9bd64bb7a09
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>