s390/mm: gmap: sort out physical vs virtual pointers usage

Fix virtual vs physical address confusion (which currently are the same).
Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20221020143159.294605-2-nrb@linux.ibm.com
Message-Id: <20221020143159.294605-2-nrb@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>