KVM: s390: allocate only one DMA page per VM

We can fit the 2k for the STFLE interpretation and the crypto
control block into one DMA page. As we now only have to allocate
one DMA page, we can clean up the code a bit.

As a nice side effect, this also fixes a problem with crycbd alignment in
case special allocation debug options are enabled, debugged by Sascha
Silbe.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>