s390/mm: force swiotlb for protected virtualization

On s390, protected virtualization guests have to use bounced I/O
buffers.  That requires some plumbing.

Let us make sure, any device that uses DMA API with direct ops correctly
is spared from the problems, that a hypervisor attempting I/O to a
non-shared page would bring.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Michael Mueller <mimu@linux.ibm.com>
Tested-by: Michael Mueller <mimu@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>