s390/mm: fix phys vs virt confusion in mark_kernel_pXd() functions family

Due to historical reasons mark_kernel_pXd() functions
misuse the notion of physical vs virtual addresses
difference.

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