lkdtm: cfi: use NULL for a null pointer rather than zero

There is a pointer being initialized with a zero, use NULL instead.

Cleans up sparse warning:
drivers/misc/lkdtm/cfi.c:100:27: warning: Using plain integer as NULL pointer

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220612202708.2754270-1-colin.i.king@gmail.com


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