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

Due to historical reasons vmem_*() functions misuse
or ignore the notion of physical vs virtual addresses
difference.

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