misc: lkdtm: Add volatile to intentional NULL pointer reference

Add a volatile qualifier where a NULL pointer is deliberately
dereferenced to trigger a panic.

Without the volatile qualifier clang will issue the following warning:
"indirection of non-volatile null pointer will be deleted,
not trap [-Wnull-dereference]" and replace the pointer reference
with a __builtin_trap() (which generates a ud2 instruction on x86_64).

Change-Id: I6ed1d723234c607eb33dc3ed0be23c093d8c835c
Link: https://lore.kernel.org/patchwork/patch/780024/
Signed-off-by: Michael Davidson <md@google.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 9e18308a5dcc2250a271e598dfe0d917b5522475
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Shadab Naseem <snaseem@codeaurora.org>

drivers/misc/lkdtm_bugs.c

@@ -67,7 +67,7 @@ void lkdtm_WARNING(void)
 
 void lkdtm_EXCEPTION(void)
 {
-	*((int *) 0) = 0;
+	*((volatile int *) 0) = 0;
 }
 
 void lkdtm_LOOP(void)