s390/pci: use virtual memory for iommu bitmap

The bitmap used to mark dma mappings can be quite large on systems
with huge amounts of memory. Use virtual memory for this bitmap.

Suggested-by: Alexander Schmidt <alexschm@de.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>