s390/smp: sort out physical vs virtual CPU0 lowcore pointer

SPX instruction called from set_prefix() expects physical
address of the lowcore to be installed, but instead the
virtual address is passed.

Note: this does not fix a bug currently, since virtual and
physical addresses are identical.

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