s390: implement ->mapping_error

s390 can also use noop_dma_ops, and while that currently does not return
errors it will so in the future.  Implementing the mapping_error method
is the proper way to have per-ops error conditions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>