s390/sysinfo: allow response buffer in normal memory

As provided with commit cd4386a9 ("s390/cpcmd,vmcp: avoid GFP_DMA
allocations") the Diagnose Code 8 response buffer does not have to be
below 2GB.

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>