s390/hypfs_diag0c: fix virtual vs physical address confusion

Add missing virt_to_phys() translation to diag0c(). This doesn't fix a
bug since virtual and physical addresses are currently the same.

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