s390/cmm: add missing virt_to_phys() conversion

diag10_range() expects a pfn, however the current cmm code is shifting
a virtual address, instead of a physical address by PAGE_SHIFT bits,
which would give a wrong result in case if V!=R.

Use virt_to_pfn() to fix this.

Note: this currently doesn't fix a real bug, since virtual addresses
are indentical to physical ones.

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