lkdtm: isolate stack corruption test

When tests were added to lkdtm that grew the stack frame, the stack
corruption test stopped working. This isolates the test in its own
function, and forces it not to be inlined.

Signed-off-by: Kees Cook <keescook@chromium.org>
Fixes: cc33c537

 ("lkdtm: add "EXEC_*" triggers")
Cc: stable <stable@vger.kernel.org> # 3.12
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>