lkdtm: fix false positive warning from -Wmaybe-uninitialized

The variable in use here doesn't matter (it's just used to exercise taking
up stack space), but this changes its use to pass its address instead,
to avoid a compiler warning:

drivers/misc/lkdtm_usercopy.c:54:15: warning: 'bad_stack' may be used uninitialized in this function [-Wmaybe-uninitialized]

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kees Cook <keescook@chromium.org>