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]