s390/smp: fix memblock_phys_free() vs memblock_free() confusion

memblock_phys_free() is used on a virtual address. Fix this by using
memblock_free().

Note: this doesn't fix a bug currently, since virtual and physical
addresses are identical.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>