swiotlb: export swiotlb_dma_ops

For architectures that do not use per-device dma ops we need to export
the dma_map_ops structure returned from get_arch_dma_ops().

Fixes: 10314e09 ("riscv: add swiotlb support")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Andreas Schwab <schwab@suse.de>

kernel/dma/swiotlb.c

@@ -1085,3 +1085,4 @@ const struct dma_map_ops swiotlb_dma_ops = {
 	.unmap_page		= swiotlb_unmap_page,
 	.dma_supported		= dma_direct_supported,
 };
+EXPORT_SYMBOL(swiotlb_dma_ops);