s390/maccess: fix absolute lowcore virtual vs physical address confusion

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

Note: this does not fix a bug currently, since virtual and physical
addresses are identical.

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