s390/setup: use memblock_free_late() to free old stack

Use memblock_free_late() to free the old machine check stack to the
buddy allocator instead of leaking it.

Fixes: b61b1595 ("s390: add stack for machine check handler")
Cc: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>