lkdtm: hide stack overflow warning for corrupt-stack test

After the latest change to make sure the compiler actually does a memset,
it is now smart enough to flag the stack overflow at compile time,
at least with gcc-7.0:

drivers/misc/lkdtm_bugs.c: In function 'lkdtm_CORRUPT_STACK':
drivers/misc/lkdtm_bugs.c:88:144: warning: 'memset' writing 64 bytes into a region of size 8 overflows the destination [-Wstringop-overflow=]

To outsmart the compiler again, this moves the memset into a noinline
function where (for now) it doesn't see that we intentionally write
broken code here.

Fixes: c55d2400 ("lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()")
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm_bugs.c

@@ -80,12 +80,17 @@ void lkdtm_OVERFLOW(void)
 	(void) recursive_loop(recur_count);
 }
 
+static noinline void __lkdtm_CORRUPT_STACK(void *stack)
+{
+	memset(stack, 'a', 64);
+}
+
 noinline void lkdtm_CORRUPT_STACK(void)
 {
 	/* Use default char array length that triggers stack protection. */
 	char data[8];
+	__lkdtm_CORRUPT_STACK(&data);
 
-	memset((void *)data, 'a', 64);
 	pr_info("Corrupted stack with '%16s'...\n", data);
 }