KVM: s390: usage hint for adapter mappings

The interface for adapter mappings was designed with code in mind
that maps each address only once; let's document this.

Otherwise, duplicate mappings are added to the list, which makes
the code ineffective and uses up the limited amount of mapping
needlessly.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>