Currently dma_map_sg_chained does not handle errors from the
underlying dma_map_sg calls.  This patch adds rollback in case
of an error by simply calling dma_unmap_sg_chained for the ones
that we've already mapped.

All current callers ignore the return value so this should have
no impact on them.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>