s390/pci_dma: split dma_update_trans

Split dma_update_trans into __dma_update_trans which handles updating
the dma translation tables and __dma_purge_tlb which takes care of
purging associated entries in the dma translation lookaside buffer.

The map_sg API makes use of this split approach by calling
__dma_update_trans once per physically contiguous address range but
__dma_purge_tlb only once per dma contiguous address range.

This results in less invocations of the expensive RPCIT instruction
when using map_sg.

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